TY - JOUR T1 - Mass ratio effects underlie ecosystem responses to environmental change JF - Journal of Ecology Y1 - 2020 A1 - M.D. Smith A1 - Koerner, S.E. A1 - Alan K. Knapp A1 - M.L. Avolio A1 - Chaves, F.A. A1 - Denton, E.M. A1 - Dietrich, J. A1 - Gibson, D.J. A1 - Gray, J. A1 - Hoffman, A.M. A1 - Hoover, D.L. A1 - Kimberly J. Komatsu A1 - Silletti, A. A1 - K.R. Wilcox A1 - Yu, Q. A1 - John M. Blair AB -

1. Random species loss has been shown experimentally to reduce ecosystem function, sometimes more than other anthropogenic environmental changes. Yet, controversy surrounds the importance of this finding for natural systems where species loss is non‐random.
2. We compiled data from 16 multi‐year experiments located at a single native tallgrass prairie site. These experiments included responses to 11 anthropogenic environmental changes, as well as non‐random biodiversity loss either the removal of uncommon/rare plant species or the most common (dominant) species.
3. As predicted by the mass ratio hypothesis, loss of a dominant species had large impacts on productivity that were comparable to other anthropogenic drivers. In contrast, the loss of uncommon/rare species had small effects on productivity despite having the largest effects on species richness.
4. The anthropogenic drivers that had the largest effects on productivity nitrogen, irrigation, and fire experienced not only loss of species but also significant changes in the abundance and identity of dominant species.
5. Synthesis. These results suggest that mass ratio effects, rather than species loss per se, are an important determinant of ecosystem function with environmental change.

VL - 108 UR - https://besjournals.onlinelibrary.wiley.com/doi/abs/10.1111/1365-2745.13330 IS - 3 ER - TY - JOUR T1 - Precipitation amount and event size interact to reduce ecosystem functioning during dry years in a mesic grassland JF - Global Change Biology Y1 - 2020 A1 - Felton, Andrew J. A1 - Slette, Ingrid J. A1 - M.D. Smith A1 - Alan K. Knapp AB -

Ongoing intensification of the hydrological cycle is altering rainfall regimes by increasing the frequency of extreme wet and dry years and the size of individual rainfall events. Despite long‐standing recognition of the importance of precipitation amount and variability for most terrestrial ecosystem processes, we lack understanding of their interactive effects on ecosystem functioning. We quantified this interaction in native grassland by experimentally eliminating temporal variability in growing season rainfall over a wide range of precipitation amounts, from extreme wet to dry conditions. We contrasted the rain use efficiency (RUE) of above‐ground net primary productivity (ANPP) under conditions of experimentally reduced versus naturally high rainfall variability using a 32‐year precipitation–ANPP dataset from the same site as our experiment. We found that increased growing season rainfall variability can reduce RUE and thus ecosystem functioning by as much as 42% during dry years, but that such impacts weaken as years become wetter. During low precipitation years, RUE is lowest when rainfall event sizes are relatively large, and when a larger proportion of total rainfall is derived from large events. Thus, a shift towards precipitation regimes dominated by fewer but larger rainfall events, already documented over much of the globe, can be expected to reduce the functioning of mesic ecosystems primarily during drought, when ecosystem processes are already compromised by low water availability.

VL - 26 UR - https://onlinelibrary.wiley.com/doi/abs/10.1111/gcb.14789 IS - 2 ER - TY - JOUR T1 - A comprehensive approach to analyzing community dynamics using rank abundance curves JF - Ecosphere Y1 - 2019 A1 - M.L. Avolio A1 - Carroll, I. A1 - Scott. L. Collins A1 - Houseman, Gregory R. A1 - Hallett, L.M. A1 - Isbell, F.L. A1 - Koerner, S.E. A1 - Kimberly J. Komatsu A1 - M.D. Smith A1 - K.R. Wilcox AB -

Univariate and multivariate methods are commonly used to explore the spatial and temporaldynamics of ecological communities, but each has limitations, including oversimplification or abstractionof communities. Rank abundance curves (RACs) potentially integrate these existing methodologies bydetailing species-level community changes. Here, we had three goals:first, to simplify analysis of commu-nity dynamics by developing a coordinated set of R functions, and second, to demystify the relationshipsamong univariate, multivariate, and RACs measures, and examine how each is influenced by the commu-nity parameters as well as data collection methods. We developed new functions for studying temporalchanges and spatial differences in RACs in an update to the R package library(“codyn”), alongside othernew functions to calculate univariate and multivariate measures of community dynamics. We also devel-oped a new approach to studying changes in the shape of RAC curves. The R package update presentedhere increases the accessibility of univariate and multivariate measures of community change over timeand difference over space. Next, we use simulated and real data to assess the RAC and multivariate mea-sures that are output from our new functions, studying (1) if they are influenced by species richness andevenness, temporal turnover, and spatial variability and (2) how the measures are related to each other.Lastly, we explore the use of the measures with an example from a long-term nutrient addition experiment.Wefind that the RAC and multivariate measures are not sensitive to species richness and evenness andthat all the measures detail unique aspects of temporal change or spatial differences. We alsofind that spe-cies reordering is the strongest correlate of a multivariate measure of compositional change and explainsmost community change observed in long-term nutrient addition experiment. Overall, we show that spe-cies reordering is potentially an understudied determinant of community changes over time or differencesbetween treatments. The functions developed here should enhance the use of RACs to further explore thedynamics of ecological communities.

VL - 10 UR - https://esajournals.onlinelibrary.wiley.com/doi/epdf/10.1002/ecs2.2881 IS - 10 ER - TY - JOUR T1 - Demystifying dominant species JF - New Phytologist Y1 - 2019 A1 - M.L. Avolio A1 - Forrestel, Elisabeth J. A1 - Chang, Cynthia C. A1 - Kimberly J. La Pierre A1 - Burghardt, Karin T. A1 - M.D. Smith AB -

The pattern of a few abundant species and many rarer species is a defining characteristic of communities worldwide. These abundant species are often referred to as dominant species. Yet, despite their importance, the term dominant species is poorly defined and often used to convey different information by different authors. Based on a review of historical and contemporary definitions we develop a synthetic definition of dominant species. This definition incorporates the relative local abundance of a species, its ubiquity across the landscape, and its impact on community and ecosystem properties. A meta‐analysis of removal studies shows that the loss of species identified as dominant by authors can significantly impact ecosystem functioning and community structure. We recommend two metrics that can be used jointly to identify dominant species in a given community and provide a roadmap for future avenues of research on dominant species. In our review, we make the case that the identity and effects of dominant species on their environments are key to linking patterns of diversity to ecosystem function, including predicting impacts of species loss and other aspects of global change on ecosystems.

VL - 223 UR - https://onlinelibrary.wiley.com/doi/abs/10.1111/nph.15789 IS - 3 ER - TY - JOUR T1 - Global change effects on plant communities are magnified by time and the number of global change factors imposed JF - Proceedings of the National Academy of Sciences Y1 - 2019 A1 - Kimberly J. Komatsu A1 - M.L. Avolio A1 - Lemoine, Nathan P. A1 - Isbell, Forest A1 - Grman, Emily A1 - Houseman, Gregory R. A1 - Koerner, Sally E. A1 - Johnson, D.S. A1 - K.R. Wilcox A1 - Juha M. Alatalo A1 - Anderson, J.P. A1 - Aerts, R. A1 - S.G. Baer A1 - Baldwin, Andrew H. A1 - Bates, J. A1 - Beierkuhnlein, C. A1 - Belote, R.T. A1 - John M. Blair A1 - Bloor, J.M.G. A1 - Bohlen, P.J. A1 - Edward W. Bork A1 - Elizabeth H. Boughton A1 - W.D. Bowman A1 - Britton, Andrea J. A1 - Cahill, James F. A1 - Chaneton, Enrique J. A1 - Chiariello, N.R. A1 - Cheng, Jimin. A1 - Scott. L. Collins A1 - Cornelissen, J.H.C. A1 - G. Du A1 - Eskelinen, Anu A1 - Firn, Jennifer A1 - Foster, B. A1 - Gough, L. A1 - Gross, K. A1 - Hallett, L.M. A1 - Han, X. A1 - Harmens, H. A1 - Hovenden, M.J. A1 - Jagerbrand, A. A1 - Jentsch, A. A1 - Kern, Christel A1 - Klanderud, Kari A1 - Alan K. Knapp A1 - Kreyling, Juergen A1 - Li, W. A1 - Luo, Yiqi A1 - McCulley, R.L. A1 - McLaren, Jennie R. A1 - Megonigal, Patrick A1 - J.W. Morgan A1 - Onipchenko, Vladimir A1 - Pennings, S.C. A1 - Prevéy, J.S. A1 - Price, Jodi N. A1 - P.B. Reich A1 - Robinson, Clare H. A1 - Russell, L.F. A1 - Sala, O.E. A1 - Seabloom, E.W. A1 - M.D. Smith A1 - Soudzilovskaia, Nadejda A. A1 - Souza, Lara A1 - K.N. Suding A1 - Suttle, B.K. A1 - Svejcar, T. A1 - Tilman, David A1 - Tognetti, P. A1 - Turkington, R. A1 - White, S. A1 - Xu, Zhuwen A1 - Yahdjian, L. A1 - Yu, Q. A1 - Zhang, Pengfei A1 - Zhang, Yunhai AB -

Global change drivers (GCDs) are expected to alter community structure and consequently, the services that ecosystems provide. Yet, few experimental investigations have examined effects of GCDs on plant community structure across multiple ecosystem types, and those that do exist present conflicting patterns. In an unprecedented global synthesis of over 100 experiments that manipulated factors linked to GCDs, we show that herbaceous plant community responses depend on experimental manipulation length and number of factors manipulated. We found that plant communities are fairly resistant to experimentally manipulated GCDs in the short term (<10 y). In contrast, long-term (≥10 y) experiments show increasing community divergence of treatments from control conditions. Surprisingly, these community responses occurred with similar frequency across the GCD types manipulated in our database. However, community responses were more common when 3 or more GCDs were simultaneously manipulated, suggesting the emergence of additive or synergistic effects of multiple drivers, particularly over long time periods. In half of the cases, GCD manipulations caused a difference in community composition without a corresponding species richness difference, indicating that species reordering or replacement is an important mechanism of community responses to GCDs and should be given greater consideration when examining consequences of GCDs for the biodiversity–ecosystem function relationship. Human activities are currently driving unparalleled global changes worldwide. Our analyses provide the most comprehensive evidence to date that these human activities may have widespread impacts on plant community composition globally, which will increase in frequency over time and be greater in areas where communities face multiple GCDs simultaneously.

VL - 116 UR - https://www.pnas.org/content/early/2019/08/14/1819027116 IS - 36 ER - TY - JOUR T1 - How ecologists define drought, and why we should do better JF - Global Change Biology Y1 - 2019 A1 - Slette, Ingrid J. A1 - Post, Alison K. A1 - Awad, Mai A1 - Even, Trevor A1 - Punzalan, Arianna A1 - Williams, Sere A1 - M.D. Smith A1 - Alan K. Knapp AB -

Drought, widely studied as an important driver of ecosystem dynamics, is predicted to increase in frequency and severity globally. To study drought, ecologists must define or at least operationalize what constitutes a drought. How this is accomplished in practice is unclear, particularly given that climatologists have long struggled to agree on definitions of drought, beyond general variants of “an abnormal deficiency of water.” We conducted a literature review of ecological drought studies (564 papers) to assess how ecologists describe and study drought. We found that ecologists characterize drought in a wide variety of ways (reduced precipitation, low soil moisture, reduced streamflow, etc.), but relatively few publications (~32%) explicitly define what are, and are not, drought conditions. More troubling, a surprising number of papers (~30%) simply equated “dry conditions” with “drought” and provided little characterization of the drought conditions studied. For a subset of these, we calculated Standardized Precipitation Evapotranspiration Index values for the reported drought periods. We found that while almost 90% of the studies were conducted under conditions quantifiable as slightly to extremely drier than average, ~50% were within the range of normal climatic variability. We conclude that the current state of the ecological drought literature hinders synthesis and our ability to draw broad ecological inferences because drought is often declared but is not explicitly defined or well characterized. We suggest that future drought publications provide at least one of the following: (a) the climatic context of the drought period based on long‐term records; (b) standardized climatic index values; (c) published metrics from drought‐monitoring organizations; (d) a quantitative definition of what the authors consider to be drought conditions for their system. With more detailed and consistent quantification of drought conditions, comparisons among studies can be more rigorous, increasing our understanding of the ecological effects of drought.

VL - 25 UR - https://onlinelibrary.wiley.com/doi/abs/10.1111/gcb.14747 IS - 10 ER - TY - JOUR T1 - Shifts in plant functional composition following long-term drought in grasslands JF - Journal of Ecology Y1 - 2019 A1 - Griffin-Nolan, Robert J. A1 - D.M. Blumenthal A1 - Scott. L. Collins A1 - Farkas, Timothy E. A1 - Hoffman, Ava M. A1 - Mueller, Kevin E. A1 - Ocheltree, Troy W. A1 - M.D. Smith A1 - Whitney, Kenneth D. A1 - Alan K. Knapp ED - Jones, Holly KW - ANPP KW - Climate change KW - community weighted traits KW - Drought KW - Functional diversity KW - plant functional traits AB -

 1. Plant traits can provide unique insights into plant performance at the community scale. Functional composition, defined by both functional diversity and community-weighted trait means (CWMs), can affect the stability of above‐ground net primary production (ANPP) in response to climate extremes. Further complexity arises, however, when functional composition itself responds to environmental change. The duration of climate extremes, such as drought, is expected to increase with rising global temperatures; thus, understanding the impacts of long-term drought on functional composition and the corresponding effect that has on ecosystem function could improve predictions of ecosystem sensitivity to climate change.
 2. We experimentally reduced growing season precipitation by 66% across six temperate grasslands for 4 years and measured changes in three indices of functional diversity (functional dispersion, richness and evenness), community-weighted trait means and phylogenetic diversity (PD). Specific leaf area (SLA), leaf nitrogen content (LNC) and (at most sites) leaf turgor loss point (πTLP) were measured for species cumulatively representing ~90% plant cover at each site.
 3. Long-term drought led to increased community functional dispersion in three sites, with negligible effects on the remaining sites. Species re-ordering following the mortality/senescence of dominant species was the main driver of increased functional dispersion. The response of functional diversity was not consistently matched by changes in phylogenetic diversity. Community-level drought strategies (assessed as CWMs) largely shifted from drought tolerance to drought avoidance and/or escape strategies, as evidenced by higher community-weighted πTLP, SLA and LNC. Lastly, ecosystem drought sensitivity (i.e. relative reduction in ANPP in drought plots) was positively correlated with community-weighted SLA and negatively correlated with functional diversity.
 4. Synthesis. Increased functional diversity following long-term drought may stabilize ecosystem functioning in response to future drought. However, shifts in community-scale drought strategies may increase ecosystem drought sensitivity, depending on the nature and timing of drought. Thus, our results highlight the importance of considering both functional diversity and abundance‐weighted traits means of plant communities as their collective effect may either stabilize or enhance ecosystem sensitivity to drought.

VL - 107 UR - https://besjournals.onlinelibrary.wiley.com/doi/10.1111/1365-2745.13252 IS - 5 ER - TY - JOUR T1 - Ambient changes exceed treatment effects on plant species abundance in long-term global change experiments JF - Glob Chang Biol Y1 - 2018 A1 - Langley, A. A1 - Chapman, S.K. A1 - Kimberly J. La Pierre A1 - M.L. Avolio A1 - W.D. Bowman A1 - Johnson, D. A1 - Isbell, F. A1 - K.R. Wilcox A1 - Foster, B. A1 - Hovenden, M. A1 - Alan K. Knapp A1 - Koerner, S.E. A1 - Lortie, C. A1 - Megonigal, J. A1 - Newton, P. A1 - Reich, B. A1 - M.D. Smith A1 - Suttle, B.K. A1 - Tilman, D. KW - elevated CO2 KW - nitrogen KW - Phosphorus KW - plant community KW - Warming KW - water AB -

The responses of species to environmental changes will determine future community composition and ecosystem function. Many syntheses of global change experiments examine the magnitude of treatment effect sizes, but we lack an understanding of how plant responses to treatments compare to ongoing changes in the unmanipulated (ambient or background) system. We used a database of long-term global change studies manipulating CO2 , nutrients, water, and temperature to answer three questions: (a) How do changes in plant species abundance in ambient plots relate to those in treated plots? (b) How does the magnitude of ambient change in species-level abundance over time relate to responsiveness to global change treatments? (c) Does the direction of species-level responses to global change treatments differ from the direction of ambient change? We estimated temporal trends in plant abundance for 791 plant species in ambient and treated plots across 16 long-term global change experiments yielding 2,116 experiment-species-treatment combinations. Surprisingly, for most species (57%) the magnitude of ambient change was greater than the magnitude of treatment effects. However, the direction of ambient change, whether a species was increasing or decreasing in abundance under ambient conditions, had no bearing on the direction of treatment effects. Although ambient communities are inherently dynamic, there is now widespread evidence that anthropogenic drivers are directionally altering plant communities in many ecosystems. Thus, global change treatment effects must be interpreted in the context of plant species trajectories that are likely driven by ongoing environmental changes.

VL - 24 UR - https://onlinelibrary.wiley.com/doi/abs/10.1111/gcb.14442 IS - 12 ER - TY - JOUR T1 - Asymmetric responses of primary productivity to altered precipitation simulated by ecosystem models across three long-term grassland sites JF - Biogeosciences Y1 - 2018 A1 - Wu, Donghai A1 - Ciais, Philippe A1 - Viovy, Nicolas A1 - Alan K. Knapp A1 - K.R. Wilcox A1 - Michael Bahn A1 - M.D. Smith A1 - Vicca, Sara A1 - Fatichi, Simone A1 - Zscheischler, Jakob A1 - He, Yue A1 - Li, Xiangyi A1 - Ito, Akihiko A1 - Arneth, Almut A1 - Harper, Anna A1 - Ukkola, Anna A1 - Paschalis, Athanasios A1 - Poulter, Benjamin A1 - Peng, Changhui A1 - Ricciuto, Daniel A1 - Reinthaler, David A1 - Chen, Guangsheng A1 - Tian, Hanqin A1 - Genet, élène A1 - Mao, Jiafu A1 - Ingrisch, Johannes A1 - Nabel, Julia E. S. M. A1 - Pongratz, Julia A1 - Lena R. Boysen A1 - Kautz, Markus A1 - Schmitt, Michael A1 - Meir, Patrick A1 - Zhu, Qiuan A1 - R. Hasibeder A1 - Sippel, Sebastian A1 - Dangal, Shree R. S. A1 - Sitch, Stephen A1 - Shi, Xiaoying A1 - Wang, Yingping A1 - Luo, Yiqi A1 - Liu, Yongwen A1 - Piao, Shilong AB -

Field measurements of aboveground net primary productivity (ANPP) in temperate grasslands suggest that both positive and negative asymmetric responses to changes in precipitation (P) may occur. Under normal range of precipitation variability, wet years typically result in ANPP gains being larger than ANPP declines in dry years (positive asymmetry), whereas increases in ANPP are lower in magnitude in extreme wet years compared to reductions during extreme drought (negative asymmetry). Whether the current generation of ecosystem models with a coupled carbon–water system in grasslands are capable of simulating these asymmetric ANPP responses is an unresolved question. In this study, we evaluated the simulated responses of temperate grassland primary productivity to scenarios of altered precipitation with 14 ecosystem models at three sites: Shortgrass steppe (SGS), Konza Prairie (KNZ) and Stubai Valley meadow (STU), spanning a rainfall gradient from dry to moist. We found that (1) the spatial slopes derived from modeled primary productivity and precipitation across sites were steeper than the temporal slopes obtained from inter-annual variations, which was consistent with empirical data; (2) the asymmetry of the responses of modeled primary productivity under normal inter-annual precipitation variability differed among models, and the mean of the model ensemble suggested a negative asymmetry across the three sites, which was contrary to empirical evidence based on filed observations; (3) the mean sensitivity of modeled productivity to rainfall suggested greater negative response with reduced precipitation than positive response to an increased precipitation under extreme conditions at the three sites; and (4) gross primary productivity (GPP), net primary productivity (NPP), aboveground NPP (ANPP) and belowground NPP (BNPP) all showed concave-down nonlinear responses to altered precipitation in all the models, but with different curvatures and mean values. Our results indicated that most models overestimate the negative drought effects and/or underestimate the positive effects of increased precipitation on primary productivity under normal climate conditions, highlighting the need for improving eco-hydrological processes in those models in the future.

VL - 15 UR - https://www.biogeosciences.net/15/3421/2018/ IS - 11 ER - TY - JOUR T1 - Carbon exchange responses of a mesic grassland to an extreme gradient of precipitation JF - Oecologia Y1 - 2018 A1 - Felton, A.J. A1 - Alan K. Knapp A1 - M.D. Smith KW - Climate extremes KW - Drought KW - Primary production KW - Rainfall KW - soil respiration AB -

Growing evidence indicates that ecosystem processes may be differentially sensitive to dry versus wet years, and that current understanding of how precipitation affects ecosystem processes may not be predictive of responses to extremes. In an experiment within a mesic grassland, we addressed this uncertainty by assessing responses of two key carbon exchange processes—aboveground net primary production (ANPP) and soil respiration (Rs) - to an extensive gradient of growing season precipitation. This gradient comprised 11 levels that specifically included extreme values in precipitation; defined as the 1st, 5th, 95th, and 99th percentiles of the 112-year climate record. Across treatments, our experimental precipitation gradient linearly increased soil moisture availability in the rooting zone (upper 20 cm). Relative to ANPP under nominal precipitation amounts (defined as between the 15th and 85th percentiles), the magnitude of ANPP responses were greatest to extreme increases in precipitation, with an underlying linear response to both precipitation and soil moisture gradients. By contrast, Rs exhibited marginally greater responses to dry versus wet extremes, with a saturating relationship best explaining responses of Rs to both precipitation and soil moisture. Our findings indicate a linear relationship between ANPP and precipitation after incorporating responses to precipitation extremes in the ANPP–precipitation relationship, yet in contrast saturating responses of Rs. As a result, current linear ANPP–precipitation relationships (up to ~ 1000 mm) within mesic grasslands appear to hold as appropriate benchmarks for ecosystems models, yet such models should incorporate nonlinearities in responses of Rs amid increased frequencies and magnitudes of precipitation extremes.

UR - https://link.springer.com/article/10.1007/s00442-018-4284-2 ER - TY - JOUR T1 - Change in dominance determines herbivore effects on plant biodiversity JF - Nature Ecology and Evolution Y1 - 2018 A1 - Koerner, S.E. A1 - M.D. Smith A1 - Burkepile, D.E. A1 - N.P. Hanan A1 - M.L. Avolio A1 - Scott. L. Collins A1 - Alan K. Knapp A1 - N.P. Lemoine A1 - E.J. Forrestel A1 - S. Eby A1 - D.I. Thompson A1 - G. Aguado-Santacruz A1 - J.P. Anderson A1 - Anderson, M. A1 - A. Angassa A1 - S. Bagchi A1 - E.S. Bakker A1 - Bastin, Gary A1 - L.E. Baur A1 - K.H. Beard A1 - E.A. Beever A1 - P.J. Bohlen A1 - Elizabeth H. Boughton A1 - Canestro, Don A1 - Cesa, Ariela A1 - Chaneton, Enrique A1 - Cheng, Jimin A1 - C.M. D’Antonio A1 - C. Deleglise A1 - Fadiala. Dembélé A1 - Josh. Dorrough A1 - David. J. Eldridge A1 - Barbara. Fernandez-Going A1 - Silvia. Fernández-Lugo A1 - Lauchlan. H. Fraser A1 - Bill. Freedman A1 - Gonzalo. García-Salgado A1 - Jacob. R. Goheen A1 - Liang. Guo A1 - Sean. Husheer A1 - Moussa. Karembé A1 - Johannes. M. H. Knops A1 - Tineke. Kraaij A1 - Andrew. Kulmatiski A1 - Minna-Maarit. Kytöviita A1 - Felipe. Lezama A1 - Gregory. Loucougaray A1 - Alejandro. Loydi, Dan G. Milchunas, A1 - Dan.G. Milchunas, A1 - Suzanne. J. Milton A1 - J.W. Morgan A1 - Claire. Moxham A1 - Kyle. C. Nehring A1 - Han. Olff A1 - Todd. M. Palmer A1 - Salvador. Rebollo A1 - Corinna. Riginos A1 - Anita. C. Risch A1 - Marta Rueda A1 - Mahesh. Sankaran A1 - Takehiro. Sasaki A1 - Kathryn. A. Schoenecker A1 - Nick. L. Schultz A1 - Martin. Schütz A1 - Angelika. Schwabe A1 - Frances. Siebert A1 - Christian. Smit A1 - Karen. A. Stahlheber A1 - Christian. Storm A1 - Dustin. J. Strong A1 - Jishuai. Su A1 - Yadugiri. V. Tiruvaimozhi A1 - Claudia. Tyler A1 - James. Val A1 - Martijn. L. Vandegehuchte A1 - Kari. E. Veblen A1 - Lance. T. Vermeire A1 - David. Ward A1 - Jianshuang. Wu A1 - Truman. P. Young A1 - Qiang. Yu A1 - Tamara. Jane. Zelikova AB -

Herbivores alter plant biodiversity (species richness) in many of the world’s ecosystems, but the magnitude and the direction of herbivore effects on biodiversity vary widely within and among ecosystems. One current theory predicts that herbivores enhance plant biodiversity at high productivity but have the opposite effect at low productivity. Yet, empirical support for the importance of site productivity as a mediator of these herbivore impacts is equivocal. Here, we synthesize data from 252 large-herbivore exclusion studies, spanning a 20-fold range in site productivity, to test an alternative hypothesis—that herbivore-induced changes in the competitive environment determine the response of plant biodiversity to herbivory irrespective of productivity. Under this hypothesis, when herbivores reduce the abundance (biomass, cover) of dominant species (for example, because the dominant plant is palatable), additional resources become available to support new species, thereby increasing biodiversity. By contrast, if herbivores promote high dominance by increasing the abundance of herbivory-resistant, unpalatable species, then resource availability for other species decreases reducing biodiversity. We show that herbivore-induced change in dominance, independent of site productivity or precipitation (a proxy for productivity), is the best predictor of herbivore effects on biodiversity in grassland and savannah sites. Given that most herbaceous ecosystems are dominated by one or a few species, altering the competitive environment via herbivores or by other means may be an effective strategy for conserving biodiversity in grasslands and savannahs globally.

VL - 2 UR - https://www.nature.com/articles/s41559-018-0696-y#article-info ER - TY - JOUR T1 - Codominant grasses differ in gene expression under experimental climate extremes in native tallgrass prairie JF - PeerJ Y1 - 2018 A1 - Hoffman, Ava M. A1 - M.L. Avolio A1 - Alan K. Knapp A1 - M.D. Smith AB -

Extremes in climate, such as heat waves and drought, are expected to become more frequent and intense with forecasted climate change. Plant species will almost certainly differ in their responses to these stressors. We experimentally imposed a heat wave and drought in the tallgrass prairie ecosystem near Manhattan, Kansas, USA to assess transcriptional responses of two ecologically important C4 grass species, Andropogon gerardii and Sorghastrum nutans. Based on previous research, we expected that S. nutans would regulate more genes, particularly those related to stress response, under high heat and drought. Across all treatments, S. nutans showed greater expression of negative regulatory and catabolism genes while A. gerardii upregulated cellular and protein metabolism. As predicted, S. nutans showed greater sensitivity to water stress, particularly with downregulation of non-coding RNAs and upregulation of water stress and catabolism genes. A. gerardii was less sensitive to drought, although A. gerardii tended to respond with upregulation in response to drought versus S. nutans which downregulated more genes under drier conditions. Surprisingly, A. gerardii only showed minimal gene expression response to increased temperature, while S. nutans showed no response. Gene functional annotation suggested that these two species may respond to stress via different mechanisms. Specifically, A. gerardii tends to maintain molecular function while S. nutans prioritizes avoidance. Sorghastrum nutans may strategize abscisic acid response and catabolism to respond rapidly to stress. These results have important implications for success of these two important grass species under a more variable and extreme climate forecast for the future.

UR - https://peerj.com/articles/4394/ ER - TY - JOUR T1 - Drought consistently alters the composition of soil fungal and bacterial communities in grasslands from two continents JF - Global Change Biology Y1 - 2018 A1 - Ochoa-Hueso, Raúl A1 - Scott. L. Collins A1 - Delgado-Baquerizo, Manuel A1 - Hamonts, Kelly A1 - Pockman, William T. A1 - Sinsabaugh, Robert L. A1 - M.D. Smith A1 - Alan K. Knapp A1 - Power, Sally A. AB -

The effects of short‐term drought on soil microbial communities remain largely unexplored, particularly at large scales and under field conditions. We used seven experimental sites from two continents (North America and Australia) to evaluate the impacts of imposed extreme drought on the abundance, community composition, richness, and function of soil bacterial and fungal communities. The sites encompassed different grassland ecosystems spanning a wide range of climatic and soil properties. Drought significantly altered the community composition of soil bacteria and, to a lesser extent, fungi in grasslands from two continents. The magnitude of the fungal community change was directly proportional to the precipitation gradient. This greater fungal sensitivity to drought at more mesic sites contrasts with the generally observed pattern of greater drought sensitivity of plant communities in more arid grasslands, suggesting that plant and microbial communities may respond differently along precipitation gradients. Actinobateria, and Chloroflexi, bacterial phyla typically dominant in dry environments, increased their relative abundance in response to drought, whereas Glomeromycetes, a fungal class regarded as widely symbiotic, decreased in relative abundance. The response of Chlamydiae and Tenericutes, two phyla of mostly pathogenic species, decreased and increased along the precipitation gradient, respectively. Soil enzyme activity consistently increased under drought, a response that was attributed to drought‐induced changes in microbial community structure rather than to changes in abundance and diversity. Our results provide evidence that drought has a widespread effect on the assembly of microbial communities, one of the major drivers of soil function in terrestrial ecosystems. Such responses may have important implications for the provision of key ecosystem services, including nutrient cycling, and may result in the weakening of plant–microbial interactions and a greater incidence of certain soil‐borne diseases.

VL - 24 UR - http://doi.wiley.com/10.1111/gcb.2018.24.issue-7 IS - 7 ER - TY - JOUR T1 - Gene expression differs in codominant prairie grasses under drought JF - Molecular Ecology Resources Y1 - 2018 A1 - Hoffman, Ava M. A1 - M.D. Smith AB -

Grasslands of the Central US are expected to experience severe droughts and other climate extremes in the future, yet we know little about how these grasses will respond in terms of gene expression. We compared gene expression in Andropogon gerardii and Sorghastrum nutans, two closely related co-dominant C4 grasses responsible for the majority of ecosystem function, using RNA-seq. We compared Trinity assemblies within each species to determine annotated functions of transcripts responding to drought. Subsequently, we compared homologous annotated gene-groups across the two species using cross-species meta-level analysis and functional clustering based on key terms. The majority of variation was found between species, as opposed to between drought and watered treatments. However, there is evidence for differential responses; Andropogon allocated gene expression differently compared to Sorghastrum, suggesting Andropogon focuses on stress alleviation (such as oxygen radical scavenging) rather than prevention. In contrast, Sorghastrum may employ a drought avoidance strategy by modulating osmotic response, especially with hormonal regulation. We found Sorghastrum tended to be more sensitive within 10 key gene-groups related to stress, abscisic acid, and trichomes, suggesting gene expression may mechanistically parallel sensitivity at the physiological level. Our findings corroborate phenotypic and physiological differences in the field, and may help explain the phenotypic mechanisms of these two species in the tallgrass prairie community under future drought scenarios.

VL - 18 UR - https://onlinelibrary.wiley.com/doi/full/10.1111/1755-0998.12733 IS - 2 ER - TY - JOUR T1 - Legacy effects of a regional drought on aboveground net primary production in six central US grasslands JF - Plant Ecology Y1 - 2018 A1 - Griffin-Nolan, Robert J. A1 - Carroll, Charles J. W. A1 - Denton, Elsie M. A1 - Johnston, Melissa K. A1 - Scott. L. Collins A1 - M.D. Smith A1 - Alan K. Knapp AB -

Global climate models predict increases in the frequency and severity of drought worldwide, directly affecting most ecosystem types. Consequently, drought legacy effects (drought-induced alterations in ecosystem function postdrought) are expected to become more common in ecosystems varying from deserts to grasslands to forests. Drought legacies in grasslands are usually negative and reduce ecosystem function, particularly after extended drought. Moreover, ecosystems that respond strongly to drought (high sensitivity) might be expected to exhibit the largest legacy effects the next year, but this relationship has not been established. We quantified legacy effects of a severe regional drought in 2012 on postdrought (2013) aboveground net primary productivity (ANPP) in six central US grasslands. We predicted that (1) the magnitude of drought legacy effects measured in 2013 would be positively related to the sensitivity of ANPP to the 2012 drought, and (2) drought legacy effects would be negative (reducing 2013 ANPP relative to that expected given normal precipitation amounts). The magnitude of legacy effects measured in 2013 was strongly related (r2 = 0.88) to the sensitivity of ANPP to the 2012 drought across these six grasslands. However, contrary to expectations, positive legacy effects (greater than expected ANPP) were more commonly observed than negative legacy effects. Thus, while the sensitivity of ANPP to drought may be a useful predictor of the magnitude of legacy effects, short-term (1-year) severe droughts may cause legacy effects that are more variable than those observed after multiyear droughts.

VL - 219 UR - http://link.springer.com/10.1007/s11258-018-0813-7.pdf IS - 5 ER - TY - JOUR T1 - Linking gene regulation, physiology, and plant biomass allocation in Andropogon gerardii in response to drought JF - Plant Ecology Y1 - 2018 A1 - M.L. Avolio A1 - Hoffman, Ava M. A1 - M.D. Smith AB -

Plant responses to drought are often initiated at the molecular level and cascade upwards to affect physiology and growth. How plants respond to and recover from drought have consequences for their growth and survival in drier climates predicted with climate change. We studied four ecologically relevant genotypes of a common C4 grass, Andropogon gerardii. These genotypes had differential responses to a decade of more variable precipitation patterns in a field experiment in native tallgrass prairie. Here, we conducted a greenhouse experiment examining how these genotypes responded to repeated 10-day drought-recovery cycles when experiencing either a severe or moderate drought. We did this twice over the course of the experiment, early, after 5 weeks, and late, after 9 weeks of drought. We studied nine genes involved in water stress signaling and drought response in leaf tissue using real-time reverse-transcriptase polymerase chain reaction (qRT-PCR). We also measured photosynthesis, stomatal conductance, and biomass accumulation and allocation. In early drought, we found consistent differences among genotypes in gene expression, leaf-level physiology, and biomass accumulation and allocation. We found genes involved in ABA, proline synthesis, and mitigating oxidative stress were differentially expressed between genotypes, while genes that coded for aquaporins and chaperones were not. In late drought, we found fewer overall differences, and little regulation of drought responsive genes. Ultimately, we found genotypes either had greater phenotypic plasticity, suggesting an ability to avoid drought and maximize water resources when they were present, or genotypes were better at tolerating drought.

VL - 219 UR - http://link.springer.com/10.1007/s11258-017-0773-3 IS - 1 ER - TY - JOUR T1 - Mean annual precipitation predicts primary production resistance and resilience to extreme drought JF - Science of The Total Environment Y1 - 2018 A1 - Haëntjens, Ellen A1 - De Boeck, Hans J. A1 - Lemoine, Nathan P. A1 - Mänd, Pille A1 - Dulay, K.G. A1 - Schmidt, Inger K. A1 - Jentsch, Anke A1 - Stampfli, Andreas A1 - Anderegg, William R.L. A1 - Michael Bahn A1 - Kreyling, Juergen A1 - Wohlgemuth, Thomas A1 - Lloret, Francisco A1 - Classen, T.Aimée. A1 - Gough, Christopher M. A1 - M.D. Smith AB -

Extreme drought is increasing in frequency and intensity in many regions globally, with uncertain consequences for the resistance and resilience of ecosystem functions, including primary production. Primary production resistance, the capacity to withstand change during extreme drought, and resilience, the degree to which production recovers, vary among and within ecosystem types, obscuring generalized patterns of ecological stability. Theory and many observations suggest forest production is more resistant but less resilient than grassland production to extreme drought; however, studies of production sensitivity to precipitation variability indicate that the processes controlling resistance and resilience may be influenced more by mean annual precipitation (MAP) than ecosystem type. Here, we conducted a global meta-analysis to investigate primary production resistance and resilience to extreme drought in 64 forests and grasslands across a broad MAP gradient. We found resistance to extreme drought was predicted by MAP; however, grasslands (positive) and forests (negative) exhibited opposing resilience relationships with MAP. Our findings indicate that common plant physiological mechanisms may determine grassland and forest resistance to extreme drought, whereas differences among plant residents in turnover time, plant architecture, and drought adaptive strategies likely underlie divergent resilience patterns. The low resistance and resilience of dry grasslands suggests that these ecosystems are the most vulnerable to extreme drought – a vulnerability that is expected to compound as extreme drought frequency increases in the future.

 

VL - 636 UR - https://doi.org/10.1016/j.scitotenv.2018.04.290 ER - TY - JOUR T1 - A reality check for climate change experiments: Do they reflect the real world? JF - Ecology Y1 - 2018 A1 - Alan K. Knapp A1 - Carroll, C.J.W. A1 - Griffin-Nolan, R.J. A1 - Slette, I.J. A1 - Chavez, F.A. A1 - Baur, L. A1 - Felton, A.J. A1 - Gray, J. A1 - Hoffman, A.M. A1 - Lemoine, N.P. A1 - Mao, W. A1 - Post, A. A1 - M.D. Smith AB -

Experiments are widely used in ecology, particularly for assessing global change impacts on ecosystem function. However, results from experiments often are inconsistent with observations made under natural conditions, suggesting the need for rigorous comparisons of experimental and observational studies. We conducted such a “reality check” for a grassland ecosystem by compiling results from nine independently conducted climate change experiments. Each experiment manipulated growing season precipitation (GSP) and measured responses in aboveground net primary production (ANPP). We compared results from experiments with long‐term (33‐yr) annual precipitation and ANPP records to ask if collectively (n = 44 experiment‐years) experiments yielded estimates of ANPP, rain‐use efficiency (RUE, grams per square meter ANPP per mm precipitation), and the relationship between GSP and ANPP comparable to observations. We found that mean ANPP and RUE from experiments did not deviate from observations. Experiments and observational data also yielded similar functional relationships between ANPP and GSP, but only within the range of historically observed GSP. Fewer experiments imposed extreme levels of GSP (outside the observed 33‐yr record), but when these were included, they altered the GSP–ANPP relationship. This result underscores the need for more experiments imposing extreme precipitation levels to resolve how forecast changes in climate regimes will affect ecosystem function in the future.

VL - 99 UR - https://esajournals.onlinelibrary.wiley.com/doi/epdf/10.1002/ecy.2474 IS - 10 ER - TY - JOUR T1 - Assessing community and ecosystem sensitivity to climate change - toward a more comparative approach JF - Journal of Vegetation Science Y1 - 2017 A1 - M.D. Smith A1 - K.R. Wilcox A1 - Power, Sally A. A1 - Tissue, David T. A1 - Alan K. Knapp AB -

Plant communities can vary widely in their sensitivity to changing precipitation regimes, as reported by Byrne et al., Mulhouse et al. and Sternberg et al. in this issue of Journal of Vegetation Science. But to understand why communities differ in their sensitivity, we argue that clearly defined metrics of sensitivity and coordinated research approaches are needed to elucidate mechanisms.

VL - 28 UR - https://onlinelibrary.wiley.com/doi/full/10.1111/jvs.12524 IS - 2 ER - TY - JOUR T1 - Asymmetric responses of primary productivity to precipitation extremes: A synthesis of grassland precipitation manipulation experiments JF - Global Change Biology Y1 - 2017 A1 - K.R. Wilcox A1 - Shi, Zheng A1 - Gherardi, Laureano A. A1 - Lemoine, Nathan P. A1 - Koerner, Sally E. A1 - Hoover, David L. A1 - Bork, Edward A1 - Byrne, Kerry M. A1 - Cahill, James A1 - Scott. L. Collins A1 - Evans, Sarah A1 - Gilgen, Anna K. A1 - Holub, Petr A1 - Jiang, Lifen A1 - Alan K. Knapp A1 - LeCain, Daniel A1 - Liang, Junyi A1 - Garcia-Palacios, Pablo A1 - Peñuelas, Josep A1 - Pockman, William T. A1 - M.D. Smith A1 - Sun, Shanghua A1 - White, Shannon R. A1 - Yahdjian, Laura A1 - Zhu, Kai A1 - Luo, Yiqi AB -

Climatic changes are altering Earth's hydrological cycle, resulting in altered precipitation amounts, increased interannual variability of precipitation, and more frequent extreme precipitation events. These trends will likely continue into the future, having substantial impacts on net primary productivity (NPP) and associated ecosystem services such as food production and carbon sequestration. Frequently, experimental manipulations of precipitation have linked altered precipitation regimes to changes in NPP. Yet, findings have been diverse and substantial uncertainty still surrounds generalities describing patterns of ecosystem sensitivity to altered precipitation. Additionally, we do not know whether previously observed correlations between NPP and precipitation remain accurate when precipitation changes become extreme. We synthesized results from 83 case studies of experimental precipitation manipulations in grasslands worldwide. We used meta-analytical techniques to search for generalities and asymmetries of aboveground NPP (ANPP) and belowground NPP (BNPP) responses to both the direction and magnitude of precipitation change. Sensitivity (i.e., productivity response standardized by the amount of precipitation change) of BNPP was similar under precipitation additions and reductions, but ANPP was more sensitive to precipitation additions than reductions; this was especially evident in drier ecosystems. Additionally, overall relationships between the magnitude of productivity responses and the magnitude of precipitation change were saturating in form. The saturating form of this relationship was likely driven by ANPP responses to very extreme precipitation increases, although there were limited studies imposing extreme precipitation change, and there was considerable variation among experiments. This highlights the importance of incorporating gradients of manipulations, ranging from extreme drought to extreme precipitation increases into future climate change experiments. Additionally, policy and land management decisions related to global change scenarios should consider how ANPP and BNPP responses may differ, and that ecosystem responses to extreme events might not be predicted from relationships found under moderate environmental changes.

VL - 23 UR - https://onlinelibrary.wiley.com/doi/abs/10.1111/gcb.13706 IS - 10 JO - Glob Change Biol ER - TY - JOUR T1 - Asynchrony among local communities stabilises ecosystem function of metacommunities JF - Ecology Letters Y1 - 2017 A1 - K.R. Wilcox A1 - Tredennick, Andrew T. A1 - Koerner, Sally E. A1 - Grman, Emily A1 - Hallett, Lauren M. A1 - M.L. Avolio A1 - Kimberly J. La Pierre A1 - Houseman, Gregory R. A1 - Isbell, Forest A1 - Johnson, David Samuel A1 - Juha M. Alatalo A1 - Baldwin, Andrew H. A1 - Edward W. Bork A1 - Elizabeth H. Boughton A1 - W.D. Bowman A1 - Britton, Andrea J. A1 - Cahill, James F. A1 - Scott. L. Collins A1 - G. Du A1 - Eskelinen, Anu A1 - Gough, Laura A1 - Jentsch, Anke A1 - Kern, Christel A1 - Klanderud, Kari A1 - Alan K. Knapp A1 - Kreyling, Juergen A1 - Luo, Yiqi A1 - McLaren, Jennie R. A1 - Megonigal, Patrick A1 - Onipchenko, Vladimir A1 - Prevéy, Janet A1 - Price, Jodi N. A1 - Robinson, Clare H. A1 - Sala, Osvaldo E. A1 - M.D. Smith A1 - Soudzilovskaia, Nadejda A. A1 - Souza, Lara A1 - Tilman, David A1 - White, Shannon R. A1 - Xu, Zhuwen A1 - Yahdjian, Laura A1 - Yu, Qiang A1 - Zhang, Pengfei A1 - Zhang, Yunhai ED - Gurevitch, Jessica KW - Alpha diversity KW - alpha variability KW - beta diversity KW - Biodiversity KW - CoRRE data base KW - patchiness KW - Plant communities KW - Primary productivity KW - species synchrony AB -

Temporal stability of ecosystem functioning increases the predictability and reliability of ecosystem services, and understanding the drivers of stability across spatial scales is important for land management and policy decisions. We used species-level abundance data from 62 plant communities across five continents to assess mechanisms of temporal stability across spatial scales. We assessed how asynchrony (i.e. different units responding dissimilarly through time) of species and local communities stabilised metacommunity ecosystem function. Asynchrony of species increased stability of local communities, and asynchrony among local communities enhanced metacommunity stability by a wide range of magnitudes (1–315%); this range was positively correlated with the size of the metacommunity. Additionally, asynchronous responses among local communities were linked with species’ populations fluctuating asynchronously across space, perhaps stemming from physical and/or competitive differences among local communities. Accordingly, we suggest spatial heterogeneity should be a major focus for maintaining the stability of ecosystem services at larger spatial scales.

UR - http://onlinelibrary.wiley.com/doi/10.1111/ele.12861/epdf ER - TY - THES T1 - Climate change impacts on population dynamics in tallgrass prairie: implications for species codominance Y1 - 2017 A1 - Gray, J.E. PB - Colorado State University CY - Fort Collins, CO VL - MS Thesis UR - https://mountainscholar.org/handle/10217/181431 ER - TY - JOUR T1 - Different clades and traits yield similar grassland functional responses JF - Proceedings of the National Academy of Sciences Y1 - 2017 A1 - Forrestel, Elisabeth J. A1 - M.J. Donoghue A1 - Edwards, Erika J. A1 - Jetz, Walter A1 - du Toit, Justin C. O. A1 - M.D. Smith KW - biogeography KW - Ecosystem function KW - Functional traits KW - grasslands. phylogenetics AB - Plant functional traits are viewed as key to predicting important ecosystem and community properties across resource gradients within and among biogeographic regions. Vegetation dynamics and ecosystem processes, such as aboveground net primary productivity (ANPP), are increasingly being modeled as a function of the quantitative traits of species, which are used as proxies for photosynthetic rates and nutrient and water-use efficiency. These approaches rely on an assumption that a certain trait value consistently confers a specific function or response under given environmental conditions. Here, we provide a critical test of this idea and evaluate whether the functional traits that drive the well-known relationship between precipitation and ANPP differ between systems with distinct biogeographic histories and species assemblages. Specifically, we compared grasslands spanning a broad precipitation gradient (∼200–1,000 mm/y) in North America and South Africa that differ in the relative representation and abundance of grass phylogenetic lineages. We found no significant difference between the regions in the positive relationship between annual precipitation and ANPP, yet the trait values underlying this relationship differed dramatically. Our results challenge the trait-based approach to predicting ecosystem function by demonstrating that different combinations of functional traits can act to maximize ANPP in a given environmental setting. Further, we show the importance of incorporating biogeographic and phylogenetic history in predicting community and ecosystem properties using traits. VL - 114 UR - https://www.pnas.org/content/114/4/705 IS - 4 ER - TY - JOUR T1 - Drought timing differentially affects above- and belowground productivity in a mesic grassland JF - Plant Ecology Y1 - 2017 A1 - Denton, Elsie M. A1 - Dietrich, John D. A1 - M.D. Smith A1 - Alan K. Knapp AB -

Climate models forecast an intensification of the global hydrological cycle with droughts becoming more frequent and severe, and shifting to times when they have been historically uncommon. Droughts, or prolonged periods of precipitation deficiency, are characteristic of most temperate grasslands, yet few experiments have explored how variation in the seasonal timing of drought may impact ecosystem function. We investigated the response of above- and belowground net primary production (ANPP & BNPP) to altered drought timing in a mesic grassland in NE Kansas. Moderate drought treatments (25% reduction from the mean growing season precipitation [GSP]) were imposed by erecting rainout shelters in late spring (LSP), early summer (ESM), and mid-summer (MSM, n = 10 plots/treatment). These treatments were compared to two controls (long-term average GSP [LTA] and ambient GSP [AMB]) and a wet treatment (+30% of the long-term average GSP [WET]). We found that LSP drought did not significantly reduce ANPP relative to control plots while the ESM and MSM drought did despite equivalent reductions in soil moisture. In contrast, the WET treatment did not affect ANPP. Neither the WET nor the drought treatments altered BNPP as compared to the controls. Our results suggest that forecasts of ecosystem responses to climate change will be improved if both the seasonal timing of alterations in precipitation as well as differential responses of above- and belowground productivity to drought are incorporated into models.

VL - 218 UR - https://link.springer.com/article/10.1007%2Fs11258-016-0690-x IS - 3 JO - Plant Ecol ER - TY - JOUR T1 - Photosynthetic responses of a dominant C4 grass to an experimental heat wave are mediated by soil moisture JF - Oecologia Y1 - 2017 A1 - Hoover, D.L. A1 - Alan K. Knapp A1 - M.D. Smith KW - Canopy temperature KW - Climate extremes KW - Drought KW - Ecophysiology KW - tallgrass prairie AB -

Extreme heat waves and drought are predicted to increase in frequency and magnitude with climate change. These extreme events often co-occur, making it difficult to separate their direct and indirect effects on important ecophysiological and carbon cycling processes such as photosynthesis. Here, we assessed the independent and interactive effects of experimental heat waves and drought on photosynthesis in Andropogon gerardii, a dominant C4 grass in a native mesic grassland. We experimentally imposed a two-week heat wave at four intensity levels under two contrasting soil moisture regimes: a well-watered control and an extreme drought. There were three main findings from this study. First, the soil moisture regimes had large effects on canopy temperature, leading to extremely high temperatures under drought and low temperatures under well-watered conditions. Second, soil moisture mediated the photosynthetic response to heat; heat reduced photosynthesis under the well-watered control, but not under the extreme drought treatment. Third, the effects of heat on photosynthesis appeared to be driven by a direct thermal effect, not indirectly through other environmental or ecophysiological variables. These results suggest that while photosynthesis in this dominant C4 grass is sensitive to heat stress, this sensitivity can be overwhelmed by extreme drought stress.

VL - 183 UR - https://link.springer.com/article/10.1007%2Fs00442-016-3755-6 IS - 1 ER - TY - JOUR T1 - Precipitation and environmental constraints on three aspects of flowering in three dominant tallgrass species JF - Functional Ecology Y1 - 2017 A1 - Lemoine, Nathan P. A1 - Dietrich, J.D. A1 - M.D. Smith AB -
  1. Flower production can comprise up to 70% of aboveground primary production in grasslands. Yet we know relatively little about how the environment and timing of rainfall determine flower productivity. Evidence suggests that deficits or additions of rainfall during phenlologically relevant periods (i.e. growth, storage, initiation of flowering, and reproduction) can determine flower production in grasslands.
  2. We used long-term data from the Konza Prairie LTER to test how fire, soil topography, and precipitation amounts during four phenologically relevant periods of the growing season constrain three aspects of flowering in three dominant C4 grass species. Specifically, we examined the probability of flowering, flowering stalk density, and individual flowering stalk biomass for Andropogon gerardii, Schizachyrium scoparium and Sorghastrum nutans.
  3. We found that each of the three species responded to the amount of precipitation during phenologically relevant periods in unique ways. All aspects of A. gerardii flowering were sensitive to precipitation during the flowering stalk elongation period (June 20 – Aug 3). The probability of S. nutans flowering was partly determined by precipitation during the rapid growth phase (April 21 – June 4), whereas flowering stalk density of this species depended on rainfall during flowering stalk elongation (June 20 – Aug 3). In contrast, all aspects of flowering of S. scoparium were relatively independent of rainfall during any period.
  4. Our results demonstrate that three functionally similar, codominant C4 grass species respond differently to phenologically relevant precipitation periods. As a result, drought during any phenological window during the growing season can adversely impact biomass and flowering production of grasslands via species-specific reductions in flowering stalk density and biomass.
UR - https://besjournals.onlinelibrary.wiley.com/doi/abs/10.1111/1365-2435.12904 ER - TY - JOUR T1 - Prospective evidence for independent nitrogen and phosphorus limitation of grasshopper (Chorthippus curtipennis) growth in a tallgrass prairie JF - PLoS One Y1 - 2017 A1 - Rode, M. A1 - Lemoine, N.P. A1 - M.D. Smith AB -

Insect herbivores play a pivotal role in regulating plant production and community composition, and their role in terrestrial ecosystems is partly determined by their feeding behavior and performance among plants of differing nutritional quality. Historically, nitrogen (N) has been considered the primary limiting nutrient of herbivorous insects, but N is only one of many potential nutrients important to insect performance. Of these nutrients, phosphorus (P) is perhaps the most important because somatic growth depends upon P-rich ribosomal RNA. Yet relatively few studies have assessed the strength of P-limitation for terrestrial insects and even fewer have simultaneously manipulated both N and P to assess the relative strengths of N- and P-limitation. Here, we tested for potential N and P limitation, as well as N:P co-limitation, on Chorthippis curtipennis (Orthoptera, Acrididae), an abundant member of arthropod communities of central US prairies. Our results demonstrate weak evidence for both N and P limitation of C. curtipennis growth rates in laboratory feeding assays. Importantly, P-limitation was just as strong as N-limitation, but we found no evidence for NP co-limitation in our study. Furthermore, nutrient limitation was not apparent in field studies, suggesting that insect growth rates may be predominately controlled by other factors, including temperature and predation. Our results suggest that P should be jointly considered, along with N, as a primary determinant of herbivore feeding behavior under both current and future climate conditions.

VL - 12 UR - https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0177754 IS - 5 ER - TY - JOUR T1 - Pushing precipitation to the extremes in distributed experiments: recommendations for simulating wet and dry years JF - Global Change Biology Y1 - 2017 A1 - Alan K. Knapp A1 - M.L. Avolio A1 - Beier, C. A1 - Carroll, C.J.W. A1 - Scott. L. Collins A1 - Dukes, J.S. A1 - Fraser, L.H. A1 - Griffin-Nolan, R.J. A1 - Hoover, D.L. A1 - Jentsch, A. A1 - Loik, M.E. A1 - Phillips, R.P. A1 - Post, A.K. A1 - Sala, O.E. A1 - Slette, I.J. A1 - Yahdjian, L. A1 - M.D. Smith AB -

Intensification of the global hydrological cycle, ranging from larger individual precipitation events to more extreme multiyear droughts, has the potential to cause widespread alterations in ecosystem structure and function. With evidence that the incidence of extreme precipitation years (defined statistically from historical precipitation records) is increasing, there is a clear need to identify ecosystems that are most vulnerable to these changes and understand why some ecosystems are more sensitive to extremes than others. To date, opportunistic studies of naturally occurring extreme precipitation years, combined with results from a relatively small number of experiments, have provided limited mechanistic understanding of differences in ecosystem sensitivity, suggesting that new approaches are needed. Coordinated distributed experiments (CDEs) arrayed across multiple ecosystem types and focused on water can enhance our understanding of differential ecosystem sensitivity to precipitation extremes, but there are many design challenges to overcome (e.g., cost, comparability, standardization). Here, we evaluate contemporary experimental approaches for manipulating precipitation under field conditions to inform the design of ‘Drought-Net’, a relatively low-cost CDE that simulates extreme precipitation years. A common method for imposing both dry and wet years is to alter each ambient precipitation event. We endorse this approach for imposing extreme precipitation years because it simultaneously alters other precipitation characteristics (i.e., event size) consistent with natural precipitation patterns. However, we do not advocate applying identical treatment levels at all sites – a common approach to standardization in CDEs. This is because precipitation variability varies >fivefold globally resulting in a wide range of ecosystem-specific thresholds for defining extreme precipitation years. For CDEs focused on precipitation extremes, treatments should be based on each site's past climatic characteristics. This approach, though not often used by ecologists, allows ecological responses to be directly compared across disparate ecosystems and climates, facilitating process-level understanding of ecosystem sensitivity to precipitation extremes.

VL - 23 UR - https://onlinelibrary.wiley.com/doi/abs/10.1111/gcb.13504 IS - 5 ER - TY - JOUR T1 - Reconciling inconsistencies in precipitation– productivity relationships: implications for climate change JF - New Phytologist Y1 - 2017 A1 - Alan K. Knapp A1 - Ciais, P. A1 - M.D. Smith AB - Precipitation (PPT) is a primary climatic determinant of plant growth and aboveground net primary production (ANPP) over much of the globe. Thus, PPT–ANPP relationships are important both ecologically and to land–atmosphere models that couple terrestrial vegetation to the global carbon cycle. Empirical PPT–ANPP relationships derived from long-term site-based data are almost always portrayed as linear, but recent evidence has accumulated that is inconsistent with an underlying linear relationship. We review, and then reconcile, these inconsistencies with a nonlinear model that incorporates observed asymmetries in PPT–ANPP relationships. Although data are currently lacking for parameterization, this new model highlights research needs that, when met, will improve our understanding of carbon cycle dynamics, as well as forecasts of ecosystem responses to climate change. VL - 214 UR - https://nph.onlinelibrary.wiley.com/doi/full/10.1111/nph.14381 IS - 1 ER - TY - JOUR T1 - Altered rainfall patterns increase forb abundance and richness in native tallgrass prairie JF - Scientific Reports Y1 - 2016 A1 - Jones, Sydney K. A1 - Scott. L. Collins A1 - John M. Blair A1 - M.D. Smith A1 - Alan K. Knapp AB -

Models predict that precipitation variability will increase with climate change. We used a 15-year precipitation manipulation experiment to determine if altering the timing and amount of growing season rainfall will impact plant community structure in annually burned, native tallgrass prairie. The altered precipitation treatment maintained the same total growing season precipitation as the ambient precipitation treatment, but received a rainfall regime of fewer, larger rain events, and longer intervals between events each growing season. Although this change in precipitation regime significantly lowered mean soil water content, overall this plant community was remarkably resistant to altered precipitation with species composition relatively stable over time. However, we found significantly higher forb cover and richness and slightly lower grass cover on average with altered precipitation, but the forb responses were manifest only after a ten-year lag period. Thus, although community structure in this grassland is relatively resistant to this type of altered precipitation regime, forb abundance in native tallgrass prairie may increase in a future characterized by increased growing season precipitation variability.

UR - https://www.nature.com/articles/srep20120 IS - 1 JO - Sci Rep ER - TY - JOUR T1 - Does ecosystem sensitivity to precipitation at the site-level conform to regional-scale predictions? JF - Ecology Y1 - 2016 A1 - K.R. Wilcox A1 - John M. Blair A1 - M.D. Smith A1 - Alan K. Knapp AB -

Central to understanding global C cycle dynamics is the functional relationship between precipitation and net primary production (NPP). At large spatial (regional) scales, the responsiveness of aboveground NPP (ANPP) to inter-annual variation in annual precipitation (AP; ANPPsensitivity) is inversely related to site-level ANPP, coinciding with turnover of plant communities along precipitation gradients. Within ecosystems experiencing chronic alterations in water availability, plant community change will also occur with unknown consequences for ANPPsensitivity. To examine the role plant community shifts may play in determining alterations in site-level ANPPsensitivity, we experimentally increased precipitation by ~35% for two decades in a native Central US grassland. Consistent with regional models, ANPPsensitivity decreased initially as water availability and ANPP increased. However, ANPPsensitivity shifted back to ambient levels when mesic species increased in abundance in the plant community. Similarly, in grassland sites with distinct mesic and xeric plant communities and corresponding 50% differences in ANPP, ANPPsensitivity did not differ over almost three decades. We conclude that responses in ANPPsensitivity to chronic alterations in water availability within an ecosystem may not conform to regional AP-ANPP patterns, despite expected changes in ANPP and plant communities. The result is unanticipated functional resistance to climate change at the site scale.

VL - 97 UR - https://esajournals.onlinelibrary.wiley.com/doi/full/10.1890/15-1437.1 ER - TY - JOUR T1 - Drivers of variation in aboveground net primary productivity and plant community composition differ across a broad precipitation gradient JF - Ecosystems Y1 - 2016 A1 - Kimberly J. La Pierre A1 - D.M. Blumenthal A1 - C.S. Brown A1 - Klein, J.A. A1 - M.D. Smith KW - dominant species KW - Great Plains KW - Nutrient Network (NutNet) KW - nutrients KW - precipitation periods KW - rare species AB -

Aboveground net primary production (ANPP) is a key integrator of C uptake and energy flow in many terrestrial ecosystems. As such, ecologists have long sought to understand the factors driving variation in this important ecosystem process. Although total annual precipitation has been shown to be a strong predictor of ANPP in grasslands across broad spatial scales, it is often a poor predictor at local scales. Here we examine the amount of variation in ANPP that can be explained by total annual precipitation versus precipitation during specific periods of the year (precipitation periods) and nutrient availability at three sites representing the major grassland types (shortgrass steppe, mixed-grass prairie, and tallgrass prairie) spanning the broad precipitation gradient of the U.S. Central Great Plains. Using observational data, we found that precipitation periods and nutrient availability were much stronger predictors of site-level ANPP than total annual precipitation. However, the specific nutrients and precipitation periods that best predicted ANPP differed among the three sites. These effects were mirrored experimentally at the shortgrass and tallgrass sites, with precipitation and nutrient availability co-limiting ANPP, but not at the mixed-grass site, where nutrient availability determined ANPP exclusive of precipitation effects. Dominant grasses drove the ANPP response to increased nutrient availability at all three sites. However, the relative responses of rare grasses and forbs were greater than those of the dominant grasses to experimental nutrient additions, thus potentially driving species turnover with chronic nutrient additions. This improved understanding of the factors driving variation in ANPP within ecosystems spanning the broad precipitation gradient of the Great Plains will aid predictions of alterations in ANPP under future global change scenarios.

VL - 19 UR - https://link.springer.com/article/10.1007%2Fs10021-015-9949-7 IS - 3 ER - TY - JOUR T1 - The effect of timing of growing season drought on flowering of a dominant C4 grass JF - Oecologia Y1 - 2016 A1 - Dietrich, J.D. A1 - M.D. Smith KW - Aboveground net primary productivity KW - Andropogon gerardii KW - Ecophysiology KW - Precipitation timing KW - tallgrass prairie AB -

Timing of precipitation is equally important as amount for determining ecosystem function, especially aboveground net primary productivity (ANPP), in a number of ecosystems. In tallgrass prairie of the Central Plains of North America, grass flowering stalks of dominant C4 grasses, such as Andropogon gerardii, can account for more than 70 % of ANPP, or almost none of it, as the number of flowering stalks produced is highly variable. Although growing season precipitation amount is important for driving variation in flowering stalk production, it remains unknown whether there are critical periods within the growing season in which sufficient rainfall must occur to allow for flowering. The effect of timing of rainfall deficit (drought) on flowering of A. gerardii, was tested by excluding rainfall during three periods within the growing season (starting in mid-April, mid-May and mid-June). Mid-summer drought (starting in mid-June) strongly reduced the flowering rate (e.g., density and biomass) of A. gerardii (e.g., as high as 94 % compared to the control), suggesting flowering is highly sensitive to precipitation at this time. This effect appeared to be related to plant water status at the time of flowering stalk initiation, rather than an indirect consequence of reduced C assimilation. Our results suggest that increased frequency of growing season drought forecast with climate change could reduce sexual reproduction in this dominant grass species, particularly if it coincides with timing of flowering stalk initiation, with important implications for ecosystem functioning.

VL - 181 UR - https://link.springer.com/article/10.1007%2Fs00442-016-3579-4 IS - 2 JO - Oecologia ER - TY - JOUR T1 - Gene expression patterns of two dominant tallgrass prairie species differ in response to warming and altered precipitation JF - Scientific Reports Y1 - 2016 A1 - M.D. Smith A1 - Hoffman, A.M. A1 - M.L. Avolio AB -

To better understand the mechanisms underlying plant species responses to climate change, we compared transcriptional profiles of the co-dominant C4 grasses, Andropogon gerardii Vitman and Sorghastrum nutans (L.) Nash, in response to increased temperatures and more variable precipitation regimes in a long-term field experiment in native tallgrass prairie. We used microarray probing of a closely related model species (Zea mays) to assess correlations in leaf temperature (Tleaf) and leaf water potential (LWP) and abundance changes of ~10,000 transcripts in leaf tissue collected from individuals of both species. A greater number of transcripts were found to significantly change in abundance levels with Tleaf and LWP in S. nutans than in A. gerardii. S. nutans also was more responsive to short-term drought recovery than A. gerardii. Water flow regulating transcripts associated with stress avoidance (e.g., aquaporins), as well as those involved in the prevention and repair of damage (e.g., antioxidant enzymes, HSPs), were uniquely more abundant in response to increasing Tleaf in S. nutans. The differential transcriptomic responses of the co-dominant C4 grasses suggest that these species may cope with and respond to temperature and water stress at the molecular level in distinct ways, with implications for tallgrass prairie ecosystem function.

VL - 6 UR - https://www.nature.com/articles/srep25522 ER - TY - JOUR T1 - The immediate and prolonged effects of climate extremes on soil respiration in a mesic grassland JF - Journal of Geophysical Research: Biogeosciences Y1 - 2016 A1 - Hoover, D.L. A1 - Alan K. Knapp A1 - M.D. Smith AB -

The predicted increase in the frequency and intensity of climate extremes is expected to impact terrestrial carbon fluxes to the atmosphere, potentially changing ecosystems from carbon sinks to sources, with positive feedbacks to climate change. As the second largest terrestrial carbon flux, soil CO2 efflux or soil respiration (Rs) is strongly influenced by soil temperature and moisture. Thus, climate extremes such as heat waves and extreme drought should have substantial impacts on Rs. We investigated the effects of such climate extremes on growing season Rs in a mesic grassland by experimentally imposing 2 years of extreme drought combined with midsummer heat waves. After this 2 year period, we continued to measure Rs during a recovery year. Two consecutive drought years reduced Rs by about 25% each growing season; however, when normal rainfall returned during the recovery year, formerly droughted plots had higher rates of Rs than control plots (up to +17%). The heat wave treatments had no effect on Rs, alone or when combined with drought, and during the growing season, soil moisture was the primary driver of Rs with little evidence for Rs temperature sensitivity. When compared to aboveground net primary production, growing season Rs was much less sensitive to drought but was more responsive postdrought. These results are consistent with the hypothesis that ecosystems become sources of CO2 during drought because carbon inputs (production) are decreased relatively more than outputs (respiration). Moreover, stimulation of Rs postdrought may lengthen the time required for net carbon exchange to return to predrought levels.

VL - 121 UR - https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/2015JG003256 IS - 4 ER - TY - JOUR T1 - Integrative modelling reveals mechanisms linking productivity and plant species richness JF - Nature Y1 - 2016 A1 - Grace, J.B. A1 - Anderson, T.M. A1 - Seabloom, E.W. A1 - E.T. Borer A1 - P. Adler A1 - Harpole, W.S. A1 - Hautier, Y. A1 - Hillebrand, H. A1 - Lind, E.M. A1 - Pärtel, M. A1 - J.D. Bakker A1 - Buckley, Y.M. A1 - Crawley, M.J. A1 - Damschen, E.I. A1 - Davies, K.F. A1 - Fay, P.A. A1 - Firn, J. A1 - Gruner, D.S. A1 - Hector, A. A1 - Knops, J.M.H. A1 - MacDougall, A.S. A1 - Melbourne, B.A. A1 - J.W. Morgan A1 - Orrock, J.L. A1 - Prober, S.M. A1 - M.D. Smith AB -

How ecosystem productivity and species richness are interrelated is one of the most debated subjects in the history of ecology1. Decades of intensive study have yet to discern the actual mechanisms behind observed global patterns2, 3. Here, by integrating the predictions from multiple theories into a single model and using data from 1,126 grassland plots spanning five continents, we detect the clear signals of numerous underlying mechanisms linking productivity and richness. We find that an integrative model has substantially higher explanatory power than traditional bivariate analyses. In addition, the specific results unveil several surprising findings that conflict with classical models4, 5, 6, 7. These include the isolation of a strong and consistent enhancement of productivity by richness, an effect in striking contrast with superficial data patterns. Also revealed is a consistent importance of competition across the full range of productivity values, in direct conflict with some (but not all) proposed models. The promotion of local richness by macroecological gradients in climatic favourability, generally seen as a competing hypothesis8, is also found to be important in our analysis. The results demonstrate that an integrative modelling approach leads to a major advance in our ability to discern the underlying processes operating in ecological systems.

VL - 529 UR - https://www.nature.com/articles/nature16524 IS - 7586 ER - TY - JOUR T1 - Nutrient additions cause divergence of tallgrass prairie plant communities resulting in loss of ecosystem stability JF - Journal of Ecology Y1 - 2016 A1 - Koerner, S.E. A1 - M.L. Avolio A1 - Kimberly J. La Pierre A1 - K.R. Wilcox A1 - M.D. Smith A1 - Scott. L. Collins AB -

1.Nitrogen (N) and phosphorus (P) deposition due to pollution and land-use change are dramatically altering biogeochemical cycles. These altered nutrient inputs affect plant communities by generally increasing dominance and reducing diversity, as well as altering community variability (heterogeneity). Less well studied are effects of changes in community variability on ecosystem functions, such as productivity, or the stability of those functions. 2.Here we use a twelve-year nutrient addition experiment in tallgrass prairie to determine variability in community responses to N and P additions and link these responses to ecosystem productivity and stability. We added two levels of N and four levels of P in a fully factorial design to 25-m2 plots in native tallgrass prairie in northeastern Kansas, USA. Each year percent cover of each species was measured in June and August in a 1-m2 subplot of each plot, and annual net primary productivity was measured in two 0.1-m2 subplots in each plot at the end of each growing season. 3.The addition of N and P together increased plant community variability across space (i.e., the replicates were significantly more different from each other in the N + P treatments than they were in the control treatment). We also found that variability of the plant community within a single plot through time increased with the addition of N alone and N and P together. The highest level of both spatial and temporal variability occurred in plots with the highest level of nutrient addition (10 g m−2 of both N and P). 4.While we found no linkage between spatial variability of community composition and the spatial stability of productivity, the temporal stability of productivity decreased with increasing temporal plant community variability. Additionally, the ability to predict the productivity response to growing season precipitation, a key environmental variable, also decreased under higher temporal community variability. 5.Synthesis. Using a 12-yr nutrient addition experiment, we found that nutrient addition leads to both spatial and temporal community variability in mesic tallgrass prairie. The changes in community variability through time were directly related to ecosystem stability. While overall shifts in community structure in response to nutrient additions are important, the change in variability of local communities has significant implications for our ability to predict how patterns of biodiversity and ecosystem function will respond to a rapidly changing world.

VL - 104 UR - https://besjournals.onlinelibrary.wiley.com/doi/full/10.1111/1365-2745.12610 ER - TY - JOUR T1 - Rangeland responses to predicted increases in drought extremity JF - Rangelands Y1 - 2016 A1 - Breshears, D.D. A1 - Alan K. Knapp A1 - Law, D.J. A1 - M.D. Smith A1 - Twidwell, D. A1 - Wonkka, C.L. KW - Drought KW - grassland KW - savanna KW - shrubland KW - state and transition KW - woodland VL - 38 UR - http://dx.doi.org/10.1016/j.rala.2016.06.009 ER - TY - JOUR T1 - Shared drivers but divergent ecological responses: Insights from long-term experiments in mesic savanna grasslands JF - BioScience Y1 - 2016 A1 - M.D. Smith A1 - Alan K. Knapp A1 - Scott. L. Collins A1 - Burkepile, D.E. A1 - Kirkman, K.P. A1 - Koerner, S.E. A1 - Thompson, D.I. A1 - John M. Blair A1 - Burns, C.E. A1 - Eby, S. A1 - Forrestel, E.J. A1 - Fynn, R.W.S. A1 - Govender, N. A1 - Hagenah, N. A1 - Hoover, D.L. A1 - K.R. Wilcox KW - Aboveground net primary productivity KW - fire KW - grassland KW - Grazing KW - plant community AB -

Fire and grazing, key determinants of structure and function of savanna grasslands worldwide, have been extensively altered by humans. We used existing long-term manipulations of fire and grazing in North American and South African mesic savanna grasslands, as well as new experiments, to determine whether the impacts of fire and grazing by large herbivores differed between these systems. We found that despite a body of literature suggesting that these savanna grasslands respond uniquely to fire and grazing, their ecosystem responses (aboveground productivity) were generally similar. In contrast, plant-community responses to fire and herbivores diverged strongly between systems. The differences in plant-community responses, as well as convergence in ecosystem function, were underpinned by a common mechanism: the response of grass dominance to changing fire and grazing regimes. As a result, divergent responses of plant communities to altered fire and grazing regimes did not preclude convergence in ecosystem function.

VL - 66 UR - https://academic.oup.com/bioscience/article/66/8/666/2464141 IS - 8 JO - BioScience ER - TY - JOUR T1 - Soil nutrient additions increase invertebrate herbivore abundances, but not herbivory, across three grassland systems JF - Oecologia Y1 - 2016 A1 - Kimberly J. La Pierre A1 - M.D. Smith KW - Central Plains KW - nitrogen KW - Per capita herbivory rate KW - Phosphorus KW - Plant tissue chemistry AB -

Resource availability may influence invertebrate communities, with important consequences for ecosystem function, such as biomass production. We assessed: (1) the effects of experimental soil nutrient additions on invertebrate abundances and feeding rates and (2) the resultant changes in the effects of invertebrates on aboveground plant biomass at three grassland sites spanning the North American Central Plains, across which plant tissue chemistry and biomass vary. Invertebrate communities and rates of herbivory were sampled within a long-term nutrient-addition experiment established at each site along the broad Central Plains precipitation gradient. Additionally, the effects of invertebrates on aboveground plant biomass were determined under ambient and elevated nutrient conditions. At the more mesic sites, invertebrate herbivore abundances increased and their per capita rate of herbivory decreased with nutrient additions. In contrast, at the semi-arid site where plant biomass is low and plant nutrient concentrations are high, invertebrate herbivore abundances did not vary and per capita rates of herbivory increased with nutrient additions. No change in the effect of invertebrate herbivores on aboveground plant biomass was observed at any of the sites. In sum, nutrient additions induced shifts in both plant biomass and leaf nutrient content, which altered invertebrate abundances and feeding rate. However, due to the inverse relationship between changes in herbivore abundance and per capita rates of herbivory, nutrient additions did not alter the effect of invertebrates on aboveground biomass. Overall, we suggest that this inverse response of herbivore abundance and per capita feeding rate may buffer ecosystems against changes in invertebrate damage in response to fluctuations in nutrient levels.

VL - 180 UR - https://link.springer.com/article/10.1007%2Fs00442-015-3471-7 IS - 2 ER - TY - JOUR T1 - Characterizing differences in precipitation regimes of extreme wet and dry years: Implications for climate change experiments JF - Global Change Biology Y1 - 2015 A1 - Alan K. Knapp A1 - D.L. Hoover A1 - K.R. Wilcox A1 - M.L. Avolio A1 - Koerner, S.E. A1 - Kimberly J. La Pierre A1 - Loik, M.E. A1 - Luo, Y. A1 - Sala, O.E. A1 - M.D. Smith KW - rainfall patterns AB -

Climate change is intensifying the hydrologic cycle and is expected to increase the frequency of extreme wet and dry years. Beyond precipitation amount, extreme wet and dry years may differ in other ways, such as the number of precipitation events, event size, and the time between events. We assessed 1614 long-term (100 year) precipitation records from around the world to identify key attributes of precipitation regimes, besides amount, that distinguish statistically extreme wet from extreme dry years. In general, in regions where mean annual precipitation (MAP) exceeded 1000 mm, precipitation amounts in extreme wet and dry years differed from average years by ~40% and 30%, respectively. The magnitude of these deviations increased to >60% for dry years and to >150% for wet years in arid regions (MAP<500 mm). Extreme wet years were primarily distinguished from average and extreme dry years by the presence of multiple extreme (large) daily precipitation events (events >99th percentile of all events); these occurred twice as often in extreme wet years compared to average years. In contrast, these large precipitation events were rare in extreme dry years. Less important for distinguishing extreme wet from dry years were mean event size and frequency, or the number of dry days between events. However, extreme dry years were distinguished from average years by an increase in the number of dry days between events. These precipitation regime attributes consistently differed between extreme wet and dry years across 12 major terrestrial ecoregions from around the world, from deserts to the tropics. Thus, we recommend that climate change experiments and model simulations incorporate these differences in key precipitation regime attributes, as well as amount into treatments. This will allow experiments to more realistically simulate extreme precipitation years and more accurately assess the ecological consequences.

VL - 21 UR - https://onlinelibrary.wiley.com/doi/abs/10.1111/gcb.12888 ER - TY - JOUR T1 - Differential sensitivity to regional-scale drought in six central US grasslands JF - Oecologia Y1 - 2015 A1 - Alan K. Knapp A1 - Carroll, C.J.W. A1 - Denton, E.M. A1 - Kimberly J. La Pierre A1 - Scott. L. Collins A1 - M.D. Smith KW - Climate change KW - Functional diversity KW - Long-term ecological research KW - Precipitation KW - Primary production AB -

Terrestrial ecosystems often vary dramatically in their responses to drought, but the reasons for this are unclear. With climate change forecasts for more frequent and extensive drought in the future, a more complete understanding of the mechanisms that determine differential ecosystem sensitivity to drought is needed. In 2012, the Central US experienced the fourth largest drought in a century, with a regional-scale 40 % reduction in growing season precipitation affecting ecosystems ranging from desert grassland to mesic tallgrass prairie. This provided an opportunity to assess ecosystem sensitivity to a drought of common magnitude in six native grasslands. We tested the prediction that drought sensitivity is inversely related to mean annual precipitation (MAP) by quantifying reductions in aboveground net primary production (ANPP). Long-term ANPP data available for each site (mean length = 16 years) were used as a baseline for calculating reductions in ANPP, and drought sensitivity was estimated as the reduction in ANPP per millimeter reduction in precipitation. Arid grasslands were the most sensitive to drought, but drought responses and sensitivity varied by more than twofold among the six grasslands, despite all sites experiencing 40 % reductions in growing season precipitation. Although drought sensitivity generally decreased with increasing MAP as predicted, there was evidence that the identity and traits of the dominant species, as well as plant functional diversity, influenced sensitivity. A more comprehensive understanding of the mechanisms leading to differences in drought sensitivity will require multi-site manipulative experiments designed to assess both biotic and abiotic determinants of ecosystem sensitivity.

VL - 177 UR - https://link.springer.com/article/10.1007%2Fs00442-015-3233-6 ER - TY - JOUR T1 - The effects of genotype richness and genomic dissimilarity of Andropogon gerardii on invasion resistance and productivity JF - Plant Ecology and Diversity Y1 - 2015 A1 - M.L. Avolio A1 - Chang, C.C. A1 - Weis, J.J. A1 - M.D. Smith KW - Andropogon bladhii KW - biodiversity ecosystem function KW - complementarity KW - dominant species KW - genomic dissimilarity KW - genotypic richness KW - invasion resistance KW - productivity KW - tallgrass prairie AB -

Background: The genetic diversity within populations has been shown to affect ecosystem functions, including productivity and invasion resistance. To date most experiments have focused on manipulation of genotypic richness and have ignored other measures of genetic diversity. Aims: In the present study we aimed to establish whether manipulated genotypic richness and genomic dissimilarity of Andropogon gerardii affect productivity and invasion resistance. Methods: We created experimental mesocosms with three levels of genotypic richness: one-, three-, and nine-genotypes. In the three-genotype treatment, we manipulated a range of genomic dissimilarity values (genetic relatedness among individuals). At the end of one growing season we measured above-ground, below-ground and total biomass of the mesocosms, and invasion resistance to Andropogon bladhii. Results: Overall, we found no significant effect of genotypic richness on any measure of ecosystem function, although there tended to be more root biomass (due to complementarity) and invasive seedling biomass with higher levels of genotypic richness. Within the three-genotype treatment we found a significant positive relationship between genomic dissimilarity and above-ground biomass, which was caused by a selection effect. We also found a positive relationship between genomic dissimilarity and biomass of A. bladhii. Conclusions: Using these two measures of genetic diversity we detected differences in the strength and mechanism of positive diversity effects within the same experiment, demonstrating the value of manipulating multiple measures of diversity when performing biodiversity–ecosystem function experiments.

VL - 8 UR - https://www.tandfonline.com/doi/abs/10.1080/17550874.2013.866990 ER - TY - JOUR T1 - Functional differences between dominant grasses drive divergent responses to large herbivore loss in mesic savanna grasslands of North America and South Africa JF - Journal of Ecology Y1 - 2015 A1 - Forrestel, E.J. A1 - M.J. Donoghue A1 - M.D. Smith KW - phylogeny AB -

Grazing and fire are disturbances integral to the evolution and maintenance of savanna grasslands. Humans are altering or completely eliminating these disturbance regimes at a global scale, with important consequences for savanna ecosystem structure and function. It is unknown whether the alteration of these disturbance regimes will have similar effects on grass communities of savanna grasslands in different geographic regions that vary in their biogeographic and evolutionary histories, as well as in the diversity of extant grazers. Here, we examined the effects of large herbivore loss on different aspects of grass community structure – taxonomic, phylogenetic and functional – across a range of fire frequencies in C4-dominated mesic savanna grassland sites of North America (Konza Prairie Biological Station, Kansas, USA) and South Africa (Kruger National Park). The goal of the study was to determine whether the loss of large herbivores exerted a consistent effect on the grass communities of two physiognomically similar grasslands with different biogeographic and grazing histories. The removal of large herbivores resulted in divergent responses in the grass communities at Konza and Kruger that was consistent across fire treatments. At Konza, there was a rapid and significant response to grazing exclusion while the response was muted and transient at Kruger. Functional syndromes associated with grazing resistance were generally conserved across sites, and it was the functional strategies of the dominant species at each site that drove the divergent responses. Further, our study supports the hypothesis that grazing and aridity may be selective forces that act in parallel as those species that were grazing resistant also occupied drier niches. Synthesis. Our study demonstrates that savanna grassland communities with different biogeographic and grazing histories respond differently to the removal of large herbivores and that climate, fire and grazing are interactive forces in maintaining savanna grassland diversity and function. We show that the functional attributes of the dominant grasses, which are in part driven by the biogeographic and grazing history experienced, are the most relevant in predicting the response of savanna ecosystems to the loss of large herbivores.

VL - 103 UR - https://besjournals.onlinelibrary.wiley.com/doi/full/10.1111/1365-2745.12376 ER - TY - JOUR T1 - Functional trait expression of grassland species shift with short- and long-term nutrient additions JF - Plant Ecology Y1 - 2015 A1 - Kimberly J. La Pierre A1 - M.D. Smith KW - Baseline species KW - Effect trait KW - Enriched species KW - Nutrient addition KW - Response trait KW - tallgrass prairie AB -

Humans are altering nutrient availability worldwide, likely affecting plant trait expression, with consequences for community composition and ecosystem function. Here, we examined the responses of plant species dominant under ambient nutrient conditions (baseline species) versus those that become dominant under increased nutrient conditions (enriched species) in a tallgrass prairie ecosystem. The expression of 8 functional traits was quantified for 3 baseline and 3 enriched species within one short-term and one long-term nutrient addition experiment. We found that enriched species occupied a trait space characterized by traits that generally correspond with faster growth rates than baseline species. Additionally, the enriched species shifted in their trait expression relative to the control more than the baseline species with nutrient additions, particularly within the long-term experiment. The trait space shifts of individual species with nutrient additions scaled up to affect community aggregate trait values within both experiments. However, traits that responded to nutrient additions at the community level were not strong predictors of aboveground net primary productivity (ANPP) within the short-term experiment. In contrast, in the long-term experiment, one response trait (community aggregate height) strongly correlated with variation in ANPP with nutrient additions. The link between plant functional traits and community and ecosystem responses to chronic nutrient additions shown here will provide important insight into key mechanisms driving grassland responses to global change.

VL - 216 UR - https://link.springer.com/article/10.1007%2Fs11258-014-0438-4 ER - TY - JOUR T1 - Global environmental change and the nature of aboveground net primary productivity responses: insights from long-term experiments JF - Oecologia Y1 - 2015 A1 - M.D. Smith A1 - Kimberly J. La Pierre A1 - Scott. L. Collins A1 - Alan K. Knapp A1 - Gross, K.L. A1 - Barrett, J.E. A1 - Frey, S.D. A1 - Gough, L. A1 - Miller, R.J. A1 - Morris, J.T. A1 - Rustad, L.E. A1 - Yarie, J. KW - Aboveground productivity KW - Hierarchical response framework (HRF) KW - Long-Term Ecological Research (LTER) Network KW - Nutrient addition KW - Precipitation manipulation AB -

Many global change drivers chronically alter resource availability in terrestrial ecosystems. Such resource alterations are known to affect aboveground net primary production (ANPP) in the short term; however, it is unknown if patterns of response change through time. We examined the magnitude, direction, and pattern of ANPP responses to a wide range of global change drivers by compiling 73 datasets from long-term (>5 years) experiments that varied by ecosystem type, length of manipulation, and the type of manipulation. Chronic resource alterations resulted in a significant change in ANPP irrespective of ecosystem type, the length of the experiment, and the resource manipulated. However, the pattern of ecosystem response over time varied with ecosystem type and manipulation length. Continuous directional responses were the most common pattern observed in herbaceous-dominated ecosystems. Continuous directional responses also were frequently observed in longer-term experiments (>11 years) and were, in some cases, accompanied by large shifts in community composition. In contrast, stepped responses were common in forests and other ecosystems (salt marshes and dry valleys) and with nutrient manipulations. Our results suggest that the response of ANPP to chronic resource manipulations can be quite variable; however, responses persist once they occur, as few transient responses were observed. Shifts in plant community composition over time could be important determinants of patterns of terrestrial ecosystem sensitivity, but comparative, long-term studies are required to understand how and why ecosystems differ in their sensitivity to chronic resource alterations.

VL - 177 UR - https://link.springer.com/article/10.1007%2Fs00442-015-3230-9 IS - 4 JO - Oecologia ER - TY - JOUR T1 - Invasibility of a mesic grassland depends on the time-scale of fluctuating resources JF - Journal of Ecology Y1 - 2015 A1 - Koerner, S.E. A1 - M.L. Avolio A1 - Chang, C.C. A1 - Grey, J. A1 - D.L. Hoover A1 - M.D. Smith AB -

1. Global change is increasing the frequency and magnitude of resource fluctuations (pulses) at multiple time-scales. According to the fluctuating resource availability hypothesis (FRAH), susceptibility of an ecosystem to invasion (i.e. invasibility) is expected to increase whenever resource supply exceeds that which is utilized by native communities. Thus, global change has the potential to increase invasibility around the world. 2. Here, we test the FRAH by adding seeds of a target invader grass species to a long-term climate change experiment manipulating precipitation pulse size in tallgrass prairie in Kansas, USA. 3. Our experimental work yielded three important findings. First, contrary to predictions of the FRAH, invasibility was reduced with short time-scale resource pulses (intra-annual time-scale). Secondly, we found evidence to suggest that at inter-annual time-scales, the FRAH is supported. Wet years resulted in an increase in the number of established seedlings as well as the number of seedlings that persisted to the end of the season. Finally, we found that invasibility was positively related to native community richness and the density of individuals in the community suggesting that native communities facilitate establishment of invader species. Perhaps more importantly, results from this 10-year invasion study also show that resource availability drives invasion and that the biotic filters of plant community structure and diversity are secondary. 4. Synthesis. Our findings suggest that intensification of precipitation regimes may enhance resistance to invasion at intra-annual time-scales, but will have opposing effects if precipitation regimes include more wet years.

VL - 103 UR - https://besjournals.onlinelibrary.wiley.com/doi/full/10.1111/1365-2745.12479 IS - 6 ER - TY - JOUR T1 - Invertebrate, not small vertebrate, herbivory interacts with nutrient availability to impact tallgrass prairie community composition and forb biomass JF - Oikos Y1 - 2015 A1 - Kimberly J. La Pierre A1 - Anthony Joern A1 - M.D. Smith AB -

The effects of herbivores and their interactions with nutrient availability on primary production and plant community composition in grassland systems is expected to vary with herbivore type. We examined the effects of invertebrate and small vertebrate herbivores and their interactions with nutrient availability on grassland plant community composition and aboveground biomass in a tallgrass prairie ecosystem. The abundance of forbs relative to grasses increased with invertebrate herbivore removals. This increase in forb abundance led to a shift in community composition, where invertebrate removals resulted in greater plant species evenness as well as a divergence in composition among plots. In contrast, vertebrate herbivore removals did not affect plant community composition or aboveground biomass. Nutrient additions alone resulted in a decrease in plant species richness and an increase in the abundance of the dominant grass, but the dominant grass species did not greatly increase in abundance when nutrient additions were combined with invertebrate removals. Rather, several subdominant forbs came to dominate the plant community. Additionally, the combined nutrient addition and invertebrate herbivore removal treatment increased forb biomass, suggesting that invertebrate herbivores suppress the responses of forb species to chronic nutrient additions. Overall, the release of forbs from invertebrate herbivore pressure may result in large shifts in species composition, with consequences for aboveground biomass and forage quality due to altered grass:forb ratios in grassland systems.

VL - 124 UR - https://onlinelibrary.wiley.com/doi/abs/10.1111/oik.01869 ER - TY - JOUR T1 - Stoichiometric homeostasis predicts plant species dominance, temporal stability and responses to global change JF - Ecology Y1 - 2015 A1 - Yu, Q. A1 - K.R. Wilcox A1 - Kimberly J. La Pierre A1 - Alan K. Knapp A1 - Han, X. A1 - M.D. Smith AB -

Why some species are consistently more abundant than others, and predicting how species will respond to global change, are fundamental questions in ecology. Long-term observations indicate that plant species with high stoichiometric homeostasis for nitrogen (HN), i.e., the ability to decouple foliar N levels from variation in soil N availability, were more common and stable through time than low HN species in a central US grassland. However, with 9-yrs of nitrogen addition, species with high HN decreased in abundance, while those with low HN increased in abundance. In contrast, in climate change experiments simulating a range of forecast hydrologic changes - extreme drought (2-yrs), increased rainfall variability (14-yrs), and chronic increases in rainfall (21-yrs) - plant species with the highest HN were least responsive to changes in soil water availability. These results suggest that HN may be predictive of plant species success and stability, and how plant species and ecosystems will respond to global-change driven alterations in resource availability.

VL - 96 UR - https://esajournals.onlinelibrary.wiley.com/doi/abs/10.1890/14-1897.1 IS - 9 ER - TY - JOUR T1 - Changes in plant community composition, not diversity, during a decade of nitrogen and phosphorus additions drive above-ground productivity in a tallgrass prairie JF - Journal of Ecology Y1 - 2014 A1 - M.L. Avolio A1 - Koerner, S.E. A1 - Kimberly J. La Pierre A1 - K.R. Wilcox A1 - G.T. Wilson A1 - M.D. Smith A1 - Scott. L. Collins AB -

Nutrient additions typically increase terrestrial ecosystem productivity, reduce plant diversity and alter plant community composition; however, the effects of P additions and interactions between N and P are understudied. We added both N (10 g m−2) and three levels of P (2.5, 5 and 10 g m−2) to a native, ungrazed tallgrass prairie burned biennially in northeastern Kansas, USA, to determine the independent and interactive effects of N and P on plant community composition and above-ground net primary productivity (ANPP). After a decade of nutrient additions, we found few effects of P alone on plant community composition, N alone had stronger effects, and N and P additions combined resulted in much larger effects than either alone. The changes in the plant community were driven by decreased abundance of C4 grasses, perhaps in response to altered interactions with mycorrhizal fungi, concurrent with increased abundance of non-N-fixing perennial and annual forbs. Surprisingly, this large shift in plant community composition had little effect on plant community richness, evenness and diversity. The shift in plant composition with N and P combined had large but variable effects on ANPP over time. Initially, N and N and P combined increased above-ground productivity of C4 grasses, but after 4 years, productivity returned to ambient levels as grasses declined in abundance and the community shifted to dominance by non-N-fixing and annual forbs. Once these forbs increased in abundance and became dominant, ANPP was more variable, with pulses in forb production only in years when the site was burned. Synthesis. We found that a decade of N and P additions interacted to drive changes in plant community composition, which had large effects on ecosystem productivity but minimal effects on plant community diversity. The large shift in species composition increased variability in ANPP over time as a consequence of the effects of burning. Thus, increased inputs of N and P to terrestrial ecosystems have the potential to alter stability of ecosystem function over time, particularly within the context of natural disturbance regimes.

VL - 102 UR - https://besjournals.onlinelibrary.wiley.com/doi/full/10.1111/1365-2745.12312 ER - TY - JOUR T1 - Contrasting sensitivities of two dominant C4 grasses to heat waves and drought JF - Plant Ecology Y1 - 2014 A1 - D.L. Hoover A1 - M.D. Smith KW - Climate change KW - Climate extremes KW - Mesic grassland KW - photosynthesis KW - productivity AB -

Heat waves and droughts are predicted to increase in frequency and intensity with climate change. However, we lack a mechanistic understanding of the independent and interactive effects of severe heat and water stress for most ecosystems. In a mesic tallgrass prairie ecosystem, we used a factorial experimental approach to assess ecophysiological and productivity responses of two dominant C4 grasses, Andropogon gerardii and Sorghastrum nutans, to a season-long drought and a mid-summer heat wave at four intensities. We hypothesized that drought would have greater impacts than heat waves, that combined effects would be greater than either factor alone, and that the dominant grasses would differ in their responses to heat and water heat stress. We detected significant reductions in photosynthesis, leaf water potential, and productivity with drought but few direct responses to the heat waves. Surprisingly, there was no additive effect of heat and water stress on any plant response. However, S. nutans was more sensitive than A. gerardii to drought. In this grassland, water stress will likely dominate photosynthetic and productivity responses caused by discrete drought and heat wave events, rather than direct or additive effects of heat stress, with differential sensitivity in these grasses altering future ecosystem structure and function.

VL - 215 UR - https://link.springer.com/article/10.1007%2Fs11258-014-0345-8 ER - TY - JOUR T1 - Convergent phylogenetic and functional responses to altered fire regimes in mesic savanna grasslands of North America and South Africa JF - New Phytologist Y1 - 2014 A1 - Forrestel, E.J. A1 - M.J. Donoghue A1 - M.D. Smith AB -

The importance of fire in the creation and maintenance of mesic grassland communities is well recognized. Improved understanding of how grasses – the dominant clade in these important ecosystems – will respond to alterations in fire regimes is needed in the face of anthropogenically driven climate and land-use change. Here, we examined how grass communities shift in response to experimentally manipulated fire regimes at multiple levels of community diversity – taxonomic, phylogenetic and functional – in C4-dominanted mesic savanna grassland sites with similar structure and physiognomy, yet disparate biogeographic histories. We found that the grass communities were similar in their phylogenetic response and aspects of their functional response to high fire frequency. Both sites exhibited phylogenetic clustering of highly abundant species in annually burned plots, driven by species of the Andropogoneae, and a narrow range of functional strategies associated with rapid post-fire regeneration in a high-light, nitrogen-limited environment. By examining multiple facets of diversity in a comparative context, we identified convergent phylogenetic and functional responses to altered fire regimes in two mesic savanna grasslands. Our results highlight the importance of a common filtering process associated with fire that is consistent across grasslands of disparate biogeographic histories and taxonomic representation.

VL - 203 UR - https://nph.onlinelibrary.wiley.com/doi/full/10.1111/nph.12846 ER - TY - JOUR T1 - Differential effects of extreme drought on production and respiration: Synthesis and modeling analysis JF - Biogeosciences Y1 - 2014 A1 - Shi, Z. A1 - Thomey, M.L. A1 - Mowll, M. A1 - Litvak, M.E. A1 - N. Brunsell A1 - Scott. L. Collins A1 - Pockman, W.T. A1 - M.D. Smith A1 - Alan K. Knapp A1 - Luo, Y. AB -

Extremes in climate may severely impact ecosystem structure and function, with both the magnitude and rate of response differing among ecosystem types and processes. We conducted a modeling analysis of the effects of extreme drought on two key ecosystem processes, production and respiration, and, to provide a broader context, we complemented this with a synthesis of published results that cover a wide variety of ecosystems. The synthesis indicated that across a broad range of biomes, gross primary production (GPP) was generally more sensitive to extreme drought (defined as proportional reduction relative to average rainfall periods) than was ecosystem respiration (ER). Furthermore, this differential sensitivity between production and respiration increased as drought severity increased; it occurred only in grassland ecosystems, and not in evergreen needle-leaf and broad-leaf forests or woody savannahs. The modeling analysis was designed to enable a better understanding of the mechanisms underlying this pattern, and focused on four grassland sites arrayed across the Great Plains, USA. Model results consistently showed that net primary productivity (NPP) was reduced more than heterotrophic respiration (Rh) by extreme drought (i.e., 67% reduction in annual ambient rainfall) at all four study sites. The sensitivity of NPP to drought was directly attributable to rainfall amount, whereas the sensitivity of Rh to drought was driven by soil drying, reduced carbon (C) input and a drought-induced reduction in soil C content – a much slower process. However, differences in reductions in NPP and Rh diminished as extreme drought continued, due to a gradual decline in the soil C pool leading to further reductions in Rh. We also varied the way in which drought was imposed in the modeling analysis; it was either imposed by simulating reductions in rainfall event size (ESR) or by reducing rainfall event number (REN). Modeled NPP and Rh decreased more by ESR than REN at the two relatively mesic sites but less so at the two xeric sites. Our findings suggest that responses of production and respiration differ in magnitude, occur on different timescales, and are affected by different mechanisms under extreme, prolonged drought.

VL - 11 UR - https://www.biogeosciences.net/11/621/2014/ ER - TY - JOUR T1 - Direct and indirect relationships between genetic diversity of a dominant grass, community diversity and above-ground productivity in tallgrass prairie JF - Journal of Vegetation Science Y1 - 2014 A1 - Chang, C.C. A1 - M.D. Smith KW - Community diversity KW - disturbance KW - dominant species KW - Fire;Grassland KW - Phylogenetic diversity KW - Plant trait KW - Productivity–diversity relationship KW - Structural equation modelling AB -

Question Both genetic diversity within a dominant species and species diversity have been shown to affect productivity. However, these two levels of diversity have the potential to affect productivity in similar or opposing ways. Our study assessed direct and indirect relationships between genetic diversity of a dominant grass species and plant community diversity on productivity within a mesic grassland with naturally co-occurring genotypes and species. Location Konza Prairie Biological Station, Kansas, USA (39°05′35″ N, 96°33′31″ W). Methods We conducted a study at two sites (~65 plots per site) in intact tallgrass prairie that have been subjected to frequent (burned 14 times) and infrequent (burned nine times) fire treatments for ~30 yrs. With frequent burning, the dominant C4 grass, Andropogon gerardii, typically has higher abundance; in contrast, infrequently burned sites typically have higher species diversity but lower abundance of A. gerardii. Therefore, we hypothesized that genetic diversity within A. gerardii would have a significant direct relationship with productivity for the frequently burned site due to higher population abundances of A. gerardii overall, whereas species diversity would have a significant direct relationship with productivity for the infrequently burned site. Results Contrary to our predictions, genetic diversity of A. gerardii was not related to above-ground productivity either directly or indirectly via traits, while diversity at the community level had a negative indirect relationship with productivity via a negative effect of A. gerardii abundance on community diversity for both frequently and infrequently burned sites. Conclusions While much of the focus of diversity–productivity research has been on direct relationships between diversity at the community or population level separately, future studies should examine the indirect and potentially interactive effects of both levels of diversity in natural communities.

VL - 25 UR - https://onlinelibrary.wiley.com/doi/abs/10.1111/jvs.12108 ER - TY - JOUR T1 - Herbivores and nutrients control grassland plant diversity via light limitation JF - Nature Y1 - 2014 A1 - E.T. Borer A1 - Seabloom, E.W. A1 - Gruner, D.S. A1 - Harpole, W.S A1 - Hillebrand, H. A1 - Lind, E.M. A1 - P. Adler A1 - J. Alberti A1 - Anderson, T.M. A1 - J.D. Bakker A1 - L.A. Biederman A1 - D.M. Blumenthal A1 - C.S. Brown A1 - Brudvig, L.A. A1 - Buckley, Y.M. A1 - Cadotte, M. A1 - Chu, C. A1 - Cleland, E.E. A1 - Crawley, M.J. A1 - Daleo, P. A1 - Damschen, E.I. A1 - Davies, K.F. A1 - DeCrappeo, N.M. A1 - G. Du A1 - Firn, J. A1 - Hautier, Y. A1 - Heckman, R.W. A1 - Hector, A. A1 - HilleRisLambers, J. A1 - Iribarne, Oscar A1 - Klein, J.A. A1 - Knops, J.M.H. A1 - Kimberly J. La Pierre A1 - Leakey, A.D.B. A1 - Li, W. A1 - MacDougall, A.S. A1 - McCulley, R.L. A1 - Melbourne, B.A. A1 - Mitchell, C.E. A1 - Joslin L. Moore A1 - Mortensen, B. A1 - O'Halloran, L.R. A1 - Orrock, J.L. A1 - Pascual, J. A1 - Prober, S.M. A1 - Pyke, D.A. A1 - A. Risch A1 - Schuetz, M. A1 - M.D. Smith A1 - Stevens, C.J. A1 - L.L. Sullivan A1 - Williams, R.J. A1 - Wragg, P.D. A1 - Wright, J.P. A1 - Yang, L.H. VL - 508 UR - https://www.nature.com/articles/nature13144 IS - 7497 ER - TY - JOUR T1 - Loss of a large grazer impacts savanna grassland plant communities similarly in North America and South Africa JF - Oecologia Y1 - 2014 A1 - Eby, S. A1 - Burkepile, D.E. A1 - Fynn, R.W.S. A1 - Burns, C.E. A1 - Govender, N. A1 - Hagenah, N. A1 - Koerner, S.E. A1 - Matchett, K.J. A1 - Thompson, D.I. A1 - K.R. Wilcox A1 - Scott. L. Collins A1 - Kirkman, K.P. A1 - Alan K. Knapp A1 - M.D. Smith KW - disturbance KW - fire KW - Grazing KW - Plant community richness KW - Species diversity AB -

Large herbivore grazing is a widespread disturbance in mesic savanna grasslands which increases herbaceous plant community richness and diversity. However, humans are modifying the impacts of grazing on these ecosystems by removing grazers. A more general understanding of how grazer loss will impact these ecosystems is hampered by differences in the diversity of large herbivore assemblages among savanna grasslands, which can affect the way that grazing influences plant communities. To avoid this we used two unique enclosures each containing a single, functionally similar large herbivore species. Specifically, we studied a bison (Bos bison) enclosure at Konza Prairie Biological Station, USA and an African buffalo (Syncerus caffer) enclosure in Kruger National Park, South Africa. Within these enclosures we erected exclosures in annually burned and unburned sites to determine how grazer loss would impact herbaceous plant communities, while controlling for potential fire-grazing interactions. At both sites, removal of the only grazer decreased grass and forb richness, evenness and diversity, over time. However, in Kruger these changes only occurred with burning. At both sites, changes in plant communities were driven by increased dominance with herbivore exclusion. At Konza, this was caused by increased abundance of one grass species, Andropogon gerardii, while at Kruger, three grasses, Themeda triandra, Panicum coloratum, and Digitaria eriantha increased in abundance.

VL - 175 UR - https://link.springer.com/article/10.1007%2Fs00442-014-2895-9 ER - TY - JOUR T1 - Plant community response to loss of large herbivores differs between North American and South African savanna grasslands JF - Ecology Y1 - 2014 A1 - Koerner, S.E. A1 - Burkepile, D.E. A1 - Fynn, R.W.S. A1 - Burns, C.E. A1 - Eby, S. A1 - Govender, N. A1 - Hagenah, N. A1 - Matchett, K.J. A1 - Thompson, D.I. A1 - K.R. Wilcox A1 - Scott. L. Collins A1 - Kirkman, K.P. A1 - Alan K. Knapp A1 - M.D. Smith AB -

Herbivory and fire shape plant community structure in grass-dominated ecosystems, but these disturbance regimes are being altered around the world. To assess the consequences of such alterations, we excluded large herbivores for seven years from mesic savanna grasslands sites burned at different frequencies in North America (Konza Prairie Biological Station, Kansas, USA) and South Africa (Kruger National Park). We hypothesized that the removal of a single grass-feeding herbivore from Konza would decrease plant community richness and shift community composition due to increased dominance by grasses. Similarly, we expected grass dominance to increase at Kruger when removing large herbivores, but because large herbivores are more diverse, targeting both grasses and forbs, at this study site, the changes due to herbivore removal would be muted. After seven years of large-herbivore exclusion, richness strongly decreased and community composition changed at Konza, whereas little change was evident at Kruger. We found that this divergence in response was largely due to differences in the traits and numbers of dominant grasses between the study sites rather than the predicted differences in herbivore assemblages. Thus, the diversity of large herbivores lost may be less important in determining plant community dynamics than the functional traits of the grasses that dominate mesic, disturbance-maintained savanna grasslands.

VL - 95 UR - https://esajournals.onlinelibrary.wiley.com/doi/full/10.1890/13-1828.1 ER - TY - JOUR T1 - Rainfall variability has minimal effects on grassland recovery from repeated grazing JF - Journal of Vegetation Science Y1 - 2014 A1 - Koerner, S.E. A1 - Scott. L. Collins A1 - John M. Blair A1 - Alan K. Knapp A1 - M.D. Smith KW - Annual net primary production KW - Climate change KW - Clipping KW - disturbance KW - diversity KW - Grass–forb interaction KW - Konza KW - Precipitation KW - Stem density AB -

Question Mesic grasslands experience a complex disturbance regime including frequent fire, grazing by large ungulates and strong inter-annual climate variability. As a result of climate change, growing season precipitation regimes are predicted to become more variable, with larger event sizes and longer dry periods resulting in more temporally dynamic soil moisture regimes. Increased climate variability is likely to interact with other disturbances, such as grazing, in grassland ecosystems. We investigated the individual and combined effects of increased rainfall variability and grazing on plant community composition, structure and function in an annually burned, native tallgrass prairie. Our overarching question was: are grazing impacts modified under a more variable precipitation regime? Location Konza Prairie, Kansas, USA. Methods Plots were established within a long-term rainfall manipulation experiment in which larger, but less frequent, rain events were imposed during the growing season without altering the total rain amount. We then simulated intense grazing pressure during one growing season by repeatedly clipping all graminoids to 5 cm and monitored recovery over 3 yr. Results Neither grazing nor rainfall treatments affected species richness; however, grazing decreased total and grass above-ground net primary production (ANPP) and increased forb ANPP relative to ungrazed plots. Grass stem density recovered from intense grazing under ambient rainfall but did not fully recover, even after 2 yr in the altered rainfall treatment. Conclusions We found that increased rainfall variability had little effect on tallgrass prairie structure and function, while grazing had large effects. Grazing and increased rainfall variability interacted to suppress grass stem density and delayed recovery relative to controls. Although stem density was reduced, individual stem size increased, resulting in no net change in ANPP. This suggests that ANPP in grazed and ungrazed North American tallgrass prairie may be relatively resilient under more temporally variable precipitation regimes.

VL - 25 UR - https://onlinelibrary.wiley.com/doi/abs/10.1111/jvs.12065 ER - TY - JOUR T1 - Resistance and resilience of a grassland ecosystem to climate extremes JF - Ecology Y1 - 2014 A1 - D.L. Hoover A1 - Alan K. Knapp A1 - M.D. Smith AB -

Climate change forecasts of more frequent climate extremes suggest that such events will become increasingly important drivers of future ecosystem dynamics and function. Because the rarity and unpredictability of naturally occurring climate extremes limits assessment of their ecological impacts, we experimentally imposed extreme drought and a mid-summer heat wave over two years in a central U.S. grassland. While the ecosystem was resistant to heat waves, it was not resistant to extreme drought, which reduced aboveground net primary productivity (ANPP) below the lowest level measured in this grassland in almost 30 years. This extreme reduction in ecosystem function was a consequence of reduced productivity of both C4 grasses and C3 forbs. However, the dominant forb was negatively impacted by the drought more than the dominant grass, and this led to a reordering of species abundances within the plant community. Although this change in community composition persisted post-drought, ANPP recovered completely the year after drought due to rapid demographic responses by the dominant grass, compensating for loss of the dominant forb. Overall, these results show that an extreme reduction in ecosystem function attributable to climate extremes (e.g., low resistance) does not preclude rapid ecosystem recovery. Given that dominance by a few species is characteristic of most ecosystems, knowledge of the traits of these species and their responses to climate extremes will be key for predicting future ecosystem dynamics and function.

VL - 95 UR - https://esajournals.onlinelibrary.wiley.com/doi/abs/10.1890/13-2186.1 ER - TY - JOUR T1 - Resource availability modulates above and belowground competitive interactions between genotypes of a dominant C4 grass JF - Functional Ecology Y1 - 2014 A1 - Chang, C.C. A1 - M.D. Smith AB -

The well-described pattern of a few common and many rare species in plant communities (dominance-diversity curves) also has been documented within populations of dominant plant species. Understanding how these common genotypes coexist has implications for how genotype richness of a dominant species may impact community and ecosystem processes. Some studies have shown that increased genotype richness of a dominant species leads to an increase in above-ground productivity, suggesting niche complementarity between genotypes. However, mechanistic understanding of how genotypes may complement one another is lacking. We conducted a pairwise competition experiment between four common and naturally co-occurring genotypes of a dominant C4 grass species, Andropogon gerardii, in tallgrass prairie of the central United States. The genotypes were grown under both intra- and intergenotypic competition with different combinations of resources (low and high light, water, and nitrogen) in the greenhouse. We determined that there were above- and below-ground phenotypic differences between genotypes which results in altered competitive interactions depending on resource conditions. Different genotypes were competitively dominant under low- and high-light conditions and low and high N and water availability. Moreover, relative yield total values (RYT) for each genotype pairwise combination indicated that all four genotypes make demands on different resources, providing evidence for niche complementarity. Finally, we found that differential success in resource acquisition, biomass accumulation, and subsequent competitive ability translated to variation in vegetative reproductive success of the genotypes, which has implications for the population dynamics of this primarily asexually reproducing perennial grass. Our results suggest that naturally co-occurring genotypes coexist because they are competitively dominant under different environmental conditions, providing insight into how genetic diversity within dominant plant species is maintained and may potentially affect important ecosystem processes.

VL - 28 UR - https://besjournals.onlinelibrary.wiley.com/doi/full/10.1111/1365-2435.12227 ER - TY - JOUR T1 - Responses to fire differ between South African and North American grassland communities JF - Journal of Vegetation Science Y1 - 2014 A1 - Kirkman, K. A1 - Scott. L. Collins A1 - M.D. Smith A1 - Alan K. Knapp A1 - Burkepile, D.E. A1 - Burns, C.E. A1 - Fynn, R.W.S. A1 - Hagenah, N. A1 - Koerner, S.E. A1 - Matchett, K.J. A1 - Thompson, D.I. A1 - K.R. Wilcox A1 - Wragg, P.D. KW - Community ecology KW - Divergence KW - Fire frequency KW - Konza Prairie Biological Station KW - Mesic grassland KW - nitrogen KW - Nutrient addition KW - Richness KW - tallgrass prairie KW - Ukulinga Research Farm AB -

Question Does fire frequency affect mesic grassland plant community structure and composition similarly in North America and South Africa? Location Konza Prairie Biological Station (KNZ), Kansas, USA, and Ukulinga Research Farm (URF), KwaZulu-Natal, South Africa. Methods Plant community structure and composition were compared among annually burned, unburned and intermediate treatments within two long-term fire frequency manipulation experiments in native grasslands in North America and South Africa using comparable methodology over a 5-yr period. Because fire may reduce soil nitrogen (N) availability and thus affect plant community structure, N additions were superimposed on the fire treatments as a means of assessing direct vs indirect mechanisms driving responses to fire. Results The total number of species was higher at URF (183) than at KNZ (57). Overall divergence in plant community response to fire frequency occurred despite similar responses to nutrient additions. At KNZ, more frequent fire resulted in dominance by a few, tall, deep-rooted rhizomatous grasses (e.g. Andropogon gerardii). On unburned sites, shorter, more shade-tolerant species such as Poa pratensis increased in abundance, although A. gerardii remained dominant. Species richness increased with decreasing fire frequency at KNZ. At URF, frequent fire resulted in short, diverse grassland weakly dominated by a range of grass species, including Themeda triandra, Tristachya leucothrix and Hyparrhenia hirta. Decreasing fire frequency reduced species richness and resulted in dominance by a few, relatively tall caespitose grasses such as Aristida junciformis. There was a complete turnover of dominant species between annually burned and unburned treatments at URF, while at KNZ A. gerardii and Sorghastrum nutans occurred across the range of treatments. N addition reduced species richness in both sites. Conclusions Different responses to fire frequency between KNZ and URF are likely linked to the dominant species and their characteristic traits, including height and method of clonal reproduction, with the rhizomatous growth form of A. gerardii dominating the North American grassland. South Africa does not have an equivalent grass species; instead, a range of tufted, non-rhizomatous species dominate across the fire frequency treatments at URF. Reductions in soil N due to frequent fire did not appear to be a common mechanism driving responses in community composition in these two grasslands.

VL - 25 UR - https://onlinelibrary.wiley.com/doi/abs/10.1111/jvs.12130 ER - TY - JOUR T1 - Coordinated distributed experiments: an emerging tool for testing global hypotheses in ecology and environmental science JF - Frontiers in Ecology and the Environment Y1 - 2013 A1 - Fraser, L.H. A1 - Henry, H.A. A1 - Carlyle, C.N. A1 - White, S.R. A1 - Beierkuhnlein, C. A1 - Cahill, J.F. A1 - Casper, B.B. A1 - Cleland, E.E. A1 - Scott. L. Collins A1 - Dukes, J.S. A1 - Alan K. Knapp A1 - Lind, E. A1 - Long, R. A1 - Luo, Y. A1 - P.B. Reich A1 - M.D. Smith A1 - Sternberg, M. A1 - Turkington, R. AB -

There is a growing realization among scientists and policy makers that an increased understanding of today's environmental issues requires international collaboration and data synthesis. Meta-analyses have served this role in ecology for more than a decade, but the different experimental methodologies researchers use can limit the strength of the meta-analytic approach. Considering the global nature of many environmental issues, a new collaborative approach, which we call coordinated distributed experiments (CDEs), is needed that will control for both spatial and temporal scale, and that encompasses large geographic ranges. Ecological CDEs, involving standardized, controlled protocols, have the potential to advance our understanding of general principles in ecology and environmental science.

VL - 11 UR - https://esajournals.onlinelibrary.wiley.com/doi/abs/10.1890/110279 ER - TY - JOUR T1 - Correlations between genetic and species diversity: effects of resource quantity and heterogeneity JF - Journal of Vegetation Science Y1 - 2013 A1 - M.L. Avolio A1 - M.D. Smith KW - Andropogon gerardii KW - C4 grass KW - dominant species KW - grassland KW - Resource heterogeneity KW - Resource quantity KW - Species genetic diversity correlation AB -

Questions It is hypothesized that species and genetic diversity are correlated because niche differentiation among species and genotypes is either affected by the same processes (positive) or each level restricts the amount of diversity in the other (negative). Although many studies have observed both positive and negative relationships, others have found no correlation between the two diversity measures. Are measures of species (richness, diversity and evenness) and genetic diversity correlated, and how does resource (soil moisture, light, nitrogen and phosphorus) quantities and heterogeneity affect both levels of diversity? Location Intact tallgrass prairie at Konza Prairie Biological Station, northeast Kansas, US. Methods We investigate the correlation between plant species and genetic diversity in a long-term precipitation manipulation experiment – the Rainfall Manipulation Plots (RaMPs) – located in intact tallgrass prairie as well as adjacent non-manipulated prairie. The RaMPs experiment has been imposing ambient and more variable precipitation regimes (a 50% increase in timing between rainfall events without changing total rainfall amount) during the growing season since 1998, resulting in reduced mean soil moisture and increased soil moisture variability. Thus, the RaMPs and non-manipulated prairie plots capture a range of soil moisture amounts and variability. Genetic diversity (measured as genotype richness and genomic dissimilarity among individuals) was quantified for the dominant grass species, Andropogon gerardii, which has large impacts on plant community structure and ecosystem function. Results We found species and genetic diversity were not significantly correlated. Genotypic richness was negatively related to soil moisture variability, but measures of species diversity were not. In the non-manipulated plots only, we found generally negative relationships between resource quantity (light and nitrogen) and community diversity, and positive relationships between resource heterogeneity (CV of light) and community diversity. Conclusions Our results suggest that a lack of a positive or negative relationship between species and genetic diversity could be due to these two levels of diversity responding differently to the identity, quantity and heterogeneity of resources.

VL - 24 UR - https://onlinelibrary.wiley.com/doi/abs/10.1111/jvs.12042 ER - TY - THES T1 - Drivers of grassland community structure and ecosystem function: the role of biotic factors in determining the ecosystem response to alterations in resource availability Y1 - 2013 A1 - Kimberly J. La Pierre PB - Yale University CY - New Haven, CT VL - PhD Dissertation UR - http://search.proquest.com/docview/1495948277?pq-origsite=gscholar ER - TY - JOUR T1 - Genetic diversity of a dominant C4 grass is altered with increased precipitation variability JF - Oecologia Y1 - 2013 A1 - M.L. Avolio A1 - Beaulieu, J. A1 - M.D. Smith KW - AFLP KW - Andropogon gerardii KW - Dice dissimilarity KW - dominant species KW - Genotypic structure AB -

Climate change has the potential to alter the genetic diversity of plant populations with consequences for community dynamics and ecosystem processes. Recent research focused on changes in climatic means has found evidence of decreased precipitation amounts reducing genetic diversity. However, increased variability in climatic regimes is also predicted with climate change, but the effects of this aspect of climate change on genetic diversity have yet to be investigated. After 10 years of experimentally increased intra-annual variability in growing season precipitation regimes, we report that the number of genotypes of the dominant C4 grass, Andropogon gerardii Vitman, has been significantly reduced in native tallgrass prairie compared with unmanipulated prairie. However, individuals showed a different pattern of genomic similarity with increased precipitation variability resulting in greater genome dissimilarity among individuals when compared to unmanipulated prairie. Further, we found that genomic dissimilarity was positively correlated with aboveground productivity in this system. The increased genomic dissimilarity among individuals in the altered treatment alongside evidence for a positive correlation of genomic dissimilarity with phenotypic variation suggests ecological sorting of genotypes may be occurring via niche differentiation. Overall, we found effects of more variable precipitation regimes on population-level genetic diversity were complex, emphasizing the need to look beyond genotype numbers for understanding the impacts of climate change on genetic diversity. Recognition that future climate change may alter aspects of genetic diversity in different ways suggests possible mechanisms by which plant populations may be able to retain a diversity of traits in the face of declining biodiversity.

VL - 171 UR - https://link.springer.com/article/10.1007%2Fs00442-012-2427-4 ER - TY - JOUR T1 - Genotypic diversity of a dominant C4 grass across a long-term fire frequency gradient JF - Journal of Plant Ecology Y1 - 2013 A1 - Chang, C.C. A1 - M.D. Smith KW - Andropogon gerardii KW - disturbance frequency KW - genetic relatedness KW - genetic-species diversity relationship KW - genotype richness KW - intermediate disturbance hypothesis KW - productivity KW - tallgrass prairie AB -

Aims and Methods Diversity-disturbance research has focused on community diversity, but disturbance frequency could impact diversity within species as well, with important consequences for community diversity and ecosystem function. We examined patterns of genetic diversity of a dominant grass species, Andropogon gerardii, in native North American tallgrass prairie sites located in eastern Kansas that have been subjected to a gradient of fire frequency treatments (burned every 1, 2, 4 or 20 years) since the 1970s. In addition, we were able to assess the relationships between genetic diversity of A. gerardii, species diversity and productivity across this range of fire frequencies. Important Findings We found no significant relationships between genetic diversity of A. gerardii at the local scale (1 m2 plot level) and disturbance frequency (burned 2 to 32 times over a 38-year period). However, at the site level (i.e. across all plots sampled within a site, ~100 m2) there were differences in genotype richness and composition, as well as genomic dissimilarity among individuals of A. gerardii. Genotype richness was greatest for the site burned at an intermediate (4-year) frequency and lowest for the infrequently (20-year) burned site. In addition, genotypes found in the frequently burned sites were more similar from each other than expected by random chance than those found in the infrequently burned sites. Genotype composition of A. gerardii was not significantly different between the frequently burned sites (annual vs. 2 year) but did differ between frequently burned and infrequently burned sites (1 and 2 year vs. 4 and 20 year, etc.). Together, these results suggest site-level ecological sorting of genotypes in intact prairie across a broad gradient of disturbance frequencies, likely driven by alterations in environmental conditions. Frequent fire promotes the abundance of dominant grass species, reduces plant community diversity and impacts ecosystem processes such as productivity. Our study suggests that genetic diversity within dominant grass species also may be affected by disturbance frequency, which could have important implications for how species are able to respond to disturbance.

VL - 6 UR - https://academic.oup.com/jpe/article/6/6/448/980977 ER - TY - JOUR T1 - Intra-specific responses of a dominant C4 grass to altered precipitation patterns JF - Plant Ecology Y1 - 2013 A1 - M.L. Avolio A1 - M.D. Smith AB -

The mechanisms by which global change alters the genotypic structure of populations by selection remain unclear. Key to this understanding is elucidating genotype–phenotype relationships under different environmental conditions as genotypes could differ in their plasticity or in their tolerance to changing environmental conditions. We have previously observed selection of certain genotypes of the dominant C4 grass Andropogon gerardii L. within the on-going Rainfall Manipulation Plots (RaMPs) experiment at Konza Prairie Biological Station in Kansas. The RaMPs experiment has been experimentally imposing ambient and more variable (altered) precipitation patterns since 1998. Here, we studied phenotypic differences among six genotypes to gain insight into what drove the pattern of selection previously observed and assess potential genotype × environmental interactions. In 2008 and 2009 we sampled individuals of genotypes in the RaMPs and within unmanipulated reference plots located adjacent to the RaMPs experiment. For each individual, we measured both leaf-level (specific leaf area, stomatal conductance) and whole-plant growth (height, biomass) traits. We consistently detected differences among genotypes in the reference plots. Additionally, when focusing on two genotypes found in the altered and ambient RaMPs we observed no genotype × environment interactions. Overall, we found in an intact population of A. gerardii there exists phenotypic variability among genotypes, but no genotype × environment interactions. Thus our results demonstrate that differences in plasticity of genotypes do no explain the pattern of selection we observed.

VL - 214 UR - http://link.springer.com/10.1007/s11258-013-0258-y ER - TY - JOUR T1 - Mechanisms of selection: Phenotypic differences among genotypes explain patterns of selection in a dominant species JF - Ecology Y1 - 2013 A1 - M.L. Avolio A1 - M.D. Smith AB -

Predicted changes in precipitation means and variability are expected to alter genotype composition of plant populations; however, it remains unclear whether selection will be for trait differences among genotypes or phenotypic plasticity. This is especially true for more variable precipitation patterns that simultaneously alter soil moisture means and variability. In a previous study we found that a decade of more variable growing-season precipitation patterns changed the genotypic composition of a dominant C4 grass population (Andropogon gerardii) in native tallgrass prairie located in northeastern Kansas. Here, we assessed potential mechanisms underlying the changes observed in population structure of this species by studying how changes in both the size and variability of watering events affected ecophysiological, growth, biomass-allocation, and fitness traits of five common genotypes of A. gerardii in a greenhouse experiment. Three of these genotypes had greater abundances or were only present in field plots receiving 10 years of greater intra-annual variability in growing-season precipitation patterns. In a fully factorial experiment, we subjected the five genotypes to three water amounts (average for Kansan study site, a 40% decrease, and a 60% increase) and two watering frequency treatments (every 5 or 10 days) to produce differences in soil moisture amount and variability, respectively. We found genotype × water amount interactions for traits related to leaf-level physiology and biomass allocation; in many cases genotypes that performed better under low soil moisture conditions were outperformed by other genotypes under high soil moisture conditions. For the three genotypes that had greater abundance in field plots that received more variable precipitation patterns, we found evidence that genotypes differed in their allocation to above- vs. belowground biomass, demonstrating phenotypic trait divergence. Our results suggest that a genetically diverse population can have enough trait variation among genotypes for adaptation to occur, and thus, for dominant species, microevolution may be an important aspect of adaptation to changing environmental conditions.

VL - 94 UR - https://esajournals.onlinelibrary.wiley.com/doi/abs/10.1890/12-1119.1 ER - TY - JOUR T1 - Predicting invasion in grassland ecosystems: is exotic dominance the real embarrassment of richness? JF - Global Change Biology Y1 - 2013 A1 - Seabloom, E.W. A1 - E.T. Borer A1 - Buckley, Y. A1 - Cleland, E.E. A1 - Davies, K. A1 - Firn, J. A1 - Harpole, W.S. A1 - Hautier, Y. A1 - Lind, E. A1 - MacDougall, A. A1 - Orrock, J.L. A1 - Prober, S.M. A1 - P. Adler A1 - J. Alberti A1 - Anderson, M.T. A1 - J.D. Bakker A1 - L.A. Biederman A1 - D.M. Blumenthal A1 - C.S. Brown A1 - Brudvig, L.A. A1 - Caldeira, M. A1 - Chu, C. A1 - Crawley, M.J. A1 - Daleo, P. A1 - Damschen, E.I. A1 - D'Antonio, C.M. A1 - DeCrappeo, N.M. A1 - Dickman, C.R. A1 - G. Du A1 - Fay, P.A. A1 - Frater, P. A1 - Gruner, D.S. A1 - Hagenah, N. A1 - Hector, A. A1 - Helm, A. A1 - Hillebrand, H. A1 - Hofmockel, K.S. A1 - Humphries, H.C. A1 - Iribarne, O. A1 - Jin, V.L. A1 - Kay, A. A1 - Kirkman, K.P. A1 - Klein, J.A. A1 - Knops, J.M.H. A1 - Kimberly J. La Pierre A1 - L. Ladwig A1 - Lambrinos, J.G. A1 - Leakey, A.D.B. A1 - Li, Q. A1 - Li, W. A1 - McCulley, R. A1 - Melbourne, B. A1 - Mitchell, C.E. A1 - Joslin L. Moore A1 - J.W. Morgan A1 - Mortensen, B. A1 - O'Halloran, L.R. A1 - Pärtel, M. A1 - Pascual, J A1 - Pyke, D.A. A1 - A. Risch A1 - Salguero-Gomez, R. A1 - Sankaran, M. A1 - Schuetz, M. A1 - Simonsen, A. A1 - M.D. Smith A1 - Stevens, C. A1 - Sullivan, L. A1 - Wardle, G.M. A1 - Wolkovich, E.M. A1 - Wragg, P.D. A1 - Wright, J. A1 - Yang, L. AB -

Invasions have increased the size of regional species pools, but are typically assumed to reduce native diversity. However, global-scale tests of this assumption have been elusive because of the focus on exotic species richness, rather than relative abundance. This is problematic because low invader richness can indicate invasion resistance by the native community or, alternatively, dominance by a single exotic species. Here, we used a globally replicated study to quantify relationships between exotic richness and abundance in grass-dominated ecosystems in 13 countries on six continents, ranging from salt marshes to alpine tundra. We tested effects of human land use, native community diversity, herbivore pressure, and nutrient limitation on exotic plant dominance. Despite its widespread use, exotic richness was a poor proxy for exotic dominance at low exotic richness, because sites that contained few exotic species ranged from relatively pristine (low exotic richness and cover) to almost completely exotic-dominated ones (low exotic richness but high exotic cover). Both exotic cover and richness were predicted by native plant diversity (native grass richness) and land use (distance to cultivation). Although climate was important for predicting both exotic cover and richness, climatic factors predicting cover (precipitation variability) differed from those predicting richness (maximum temperature and mean temperature in the wettest quarter). Herbivory and nutrient limitation did not predict exotic richness or cover. Exotic dominance was greatest in areas with low native grass richness at the site- or regional-scale. Although this could reflect native grass displacement, a lack of biotic resistance is a more likely explanation, given that grasses comprise the most aggressive invaders. These findings underscore the need to move beyond richness as a surrogate for the extent of invasion, because this metric confounds monodominance with invasion resistance. Monitoring species' relative abundance will more rapidly advance our understanding of invasions.

VL - 19 UR - https://onlinelibrary.wiley.com/doi/abs/10.1111/gcb.12370 IS - 12 ER - TY - JOUR T1 - Community stability does not preclude ecosystem sensitivity to chronic resource alteration JF - Functional Ecology Y1 - 2012 A1 - Alan K. Knapp A1 - J. M. Briggs A1 - M.D. Smith VL - 26 UR - https://besjournals.onlinelibrary.wiley.com/doi/full/10.1111/j.1365-2435.2012.02053.x ER - TY - THES T1 - Dimensions of diversity and their direct and indirect effects on tallgrass prairie ecosystem functioning Y1 - 2012 A1 - Chang, C.C. AB -

Understanding the role of biological diversity to ecosystem function in ecological communities is particularly important now in the face of unprecedented, human-driven global change. Anthropogenic impacts such as climate change, nitrogen deposition, and land-use changes that modify natural disturbance regimes and promote the introduction of invasive species are all altering environmental conditions in such a way as to impact both diversity and ecosystem functioning. Diversity can be described in several ways and until recently, most research has focused on the relationship between the number and diversity of species and ecosystem function. Recently however, there has been a growing focus on the importance of genetic diversity within a dominant species and its impact on community and ecosystem processes. In this dissertation, I examine the relationship between community and population diversity, evaluate how diversity at both levels is maintained under varying environmental conditions, and test the relative effects of population and community level diversity on ecosystem function. First, I utilized a long-term experimental burn manipulation to examine the impact of fire frequency on genetic diversity of the dominant species and assessed the relationship between genetic diversity of the dominant species, species diversity, and productivity across the entire fire gradient. Genetic diversity of the dominant species was highest in a four year burn cycle and lowest in an annual and two year burn cycle. However, there was not a significant relationship between genetic and species diversity or between genetic diversity of the dominant species and productivity across the disturbance Gradient. In the second and third study, I explored in further detail the relationship between genetic diversity of the dominant species, species diversity, and evaluate the direct and indirect effects of both levels of diversity on productivity and invasion resistance. Contrary to expectations, genetic diversity of the dominant species did not have a direct effect on invasion resistance or productivity. Rather the effects of diversity at both the community and population level are primarily indirect via traits. Genetic differences between naturally co-occurring genotypes of the perennial dominant grass species were more subtle, and thus also highlighted the necessity of conducting studies in intact communities with naturally co-occurring individuals. Finally, to tease apart the possible mechanisms for how diversity within the population is maintained and subsequently impacts community and ecosystem processes, I conducted a pairwise competition experiment with the four most common genotypes of the dominant species and manipulated light, water, and nitrogen resources. Phenotypic differences between genotypes resulted in different competitive successes depending on resource conditions. Some genotypes were able to take advantage of increased water or nitrogen resources under low and high light conditions, providing evidence for niche complementarity. There was evidence that some genotypes were more antagonistic under low versus high resource conditions while other genotypes were more synergistic. This differential success in resource acquisition, biomass accumulation, and subsequent competitive ability translated to variation in vegetative reproductive success, which has implications for the population dynamics of this perennial, primarily asexually reproductive, dominant C4 grass. Together the results from these four studies suggest that the roles of population and community level diversity are complex in intact, natural tallgrass prairie where diversity effects on ecosystem function may be primarily indirect via traits.

PB - Yale University CY - New Haven, CT VL - PhD Dissertation UR - http://search.proquest.com/docview/1039554547 ER - TY - THES T1 - Genetic diversity of Andropogon gerardii: Impacts of altered precipitation patterns on a dominant species Y1 - 2012 A1 - M.L. Avolio AB -

Global change is expected to shift climatic regimes and cause whole communities of organisms to experience novel environments. Key aspects of forecast climate change are alterations in both the amount of precipitation and the variability of precipitation regimes. The ability of a population to adapt will depend on the genetic diversity of the population, and the traits of the genotypes present in the population. Here, I assess the response a dominant C4 tallgrass population to novel environmental conditions resulting from forecast climate change by studying whether more variable precipitation patterns affect the genetic diversity of Andropogon gerardii. I then investigate the potential mechanisms driving the observed response. Ultimately, the way in which population genetic diversity is affected by more variable precipitation patterns will shed insight into how this important species will adapt to future climate change. First, I developed the tools to study the genetic diversity of A. gerardii at the plant neighborhood scale, the scale at which individuals compete for resources. I designed amplified fragment length polymorphism primers for A. gerardii and determined the appropriate scale at which to sample individuals to accurately capture genetic diversity. I also examined how genetic diversity is measured in ecological studies by comparing genotype-based measures and genome-based measures of diversity. Ultimately, I argue that genome-based measures should be included in future studies alongside genotypic-based measures because they capture a greater spectrum of genetic differences among individuals. Next, using what I established for studying genetic diversity, I examined how a decade of altered precipitation patterns affected the genetic diversity of A. gerardii. To do this I worked within the Rainfall Manipulation Plots (RaMPs) experiment at Konza Prairie Biological Station in Kansas, which experimentally imposes ambient and more variable precipitation patterns. After ten years of experimentally increased intra-annual variability in growing season precipitation regimes, I report that the number of genotypes of the dominant C4 grass, Andropogon gerardii Vitman, has been significantly reduced in native tallgrass prairie compared with unmanipulated prairie. However, individuals showed a different pattern of genomic similarity with increased precipitation variability – there was greater genome dissimilarity among individuals when compared to unmanipulated prairie. In my next two chapters, I aim to understand the mechanism that drove the observed shift in genetic diversity. First, I studied phenotypic differences among six common genotypes of Andropogon gerardii across three different environmental conditions to study genotype × environmental interactions. I consistently detected differences among the focal genotypes for all traits measured across environmental treatments, however, I observed no genotype × year interactions, and phenotypic differences among genotypes were diminished within environmental conditions. To assess potential mechanisms underlying the changes observed in population structure of this species, I continued to study five of the same genotypes, three of which had greater abundances or were only present in plots receiving more variable rainfall patterns. In a greenhouse study, I investigated how both changes in the size and variability of watering events affected ecophysiological, growth, biomass allocation and fitness traits. I found genotype × water amount interactions for traits related to leaf level physiology and biomass allocation; genotypes that performed better under low soil moisture conditions were outperformed by other genotypes under high soil moisture conditions. For the three genotypes that had greater abundance in plots that received a decade of altered rainfall regimes, I found evidence of phenotypic trait divergence as well as greater plasticity for ecophysiological traits. Lastly, I investigated correlations between species and genetic diversity in tallgrass prairie across an experimental manipulation of soil moisture. I found species and genetic diversity were not correlated, and that genotypic richness was negatively related to soil moisture variability, but measures of species diversity were not related to soil moisture. My results suggest that a lack relationship between species and genetic diversity at the populations scale could be because species and genetic diversity are responding differently to environmental resources. Ultimately, my dissertation is an in-depth examination of how environmental conditions affect the genetic diversity of a dominant species. I found that for genetically diverse species, such as dominant species, microevolution might be an important aspect of adaptation to novel environmental conditions experienced with climate change.

PB - Yale University CY - New Haven, CT VL - PhD Dissertation UR - http://search.proquest.com/docview/1272028956 ER - TY - JOUR T1 - Invasion of an intact plant community: the role of population vs. community level diversity JF - Oecologia Y1 - 2012 A1 - Chang, C.C. A1 - M.D. Smith KW - Andropogon bladhii KW - dominant species KW - genetic diversity KW - grassland KW - tallgrass prairie AB -

To improve the understanding of how native plant diversity influences invasion, we examined how population and community diversity may directly and indirectly be related to invasion in a natural field setting. Due to the large impact of the dominant C4 grass species (Andropogon gerardii) on invasion resistance of tallgrass prairie, we hypothesized that genetic diversity and associated traits within a population of this species would be more strongly related to invasion than diversity or traits of the rest of the community. We added seeds of the exotic invasive C4 grass, A. bladhii, to 1-m2 plots in intact tallgrass prairie that varied in genetic diversity of A. gerardii and plant community diversity, but not species richness. We assessed relationships among genetic diversity and traits of A. gerardii, community diversity, community aggregated traits, resource availability, and early season establishment and late-season persistence of the invader using structural equation modeling (SEM). SEM models suggested that community diversity likely enhanced invasion indirectly through increasing community aggregated specific leaf area as a consequence of more favorable microclimatic conditions for seedling establishment. In contrast, neither population nor community diversity was directly or indirectly related to late season survival of invasive seedlings. Our research suggests that while much of diversity–invasion research has separately focused on the direct effects of genetic and species diversity, when taken together, we find that the role of both levels of diversity on invasion resistance may be more complex, whereby effects of diversity may be primarily indirect via traits and vary depending on the stage of invasion.

VL - 168 UR - https://link.springer.com/article/10.1007%2Fs00442-011-2157-z ER - TY - JOUR T1 - Measuring genetic diversity in ecological studies JF - Plant Ecology Y1 - 2012 A1 - M.L. Avolio A1 - Beaulieu, J. A1 - Lo, E. A1 - M.D. Smith KW - Community diversity KW - Ecosystem function KW - Genome diversity KW - Phenotype KW - SGDC AB -

There is an increasing interest in how genetic diversity may correlate with and influence community and ecosystem properties. Genetic diversity can be defined in multiple ways, and currently lacking in ecology is a consensus on how to measure genetic diversity. Here, we examine two broad classes of genetic diversity: genotype-based and genome-based measures. Genotype-based measures, such as genotypic richness, are more commonly used in ecological studies, and often it is assumed that as genotypic diversity increases, genomic diversity (the number of genetic polymorphisms and/or genomic dissimilarity among individuals) also increases. However, this assumption is rarely assessed. We tested this assumption by investigating correlations between genotype- and genome-based measures of diversity using two plant population genetic datasets: one observational with data collected at Konza Prairie, KS, and the other based on simulated populations with five levels of genotypic richness, a typical design of genetic diversity experiments. We found conflicting results for both datasets; we found a mismatch between genotypic and genomic diversity measures for the field data, but not the simulated data. Last, we tested the consequences of this mismatch and found that correlations between genetic diversity and community/ecosystem properties depended on metric used. Ultimately, we argue that genome-based measures should be included in future studies alongside genotypic-based measures because they capture a greater spectrum of genetic differences among individuals.

VL - 213 UR - https://link.springer.com/article/10.1007%2Fs11258-012-0069-6 ER - TY - JOUR T1 - Past, present, and future roles of long-term experiments in the LTER Network JF - Bioscience Y1 - 2012 A1 - Alan K. Knapp A1 - M.D. Smith A1 - Hobbie, S.E. A1 - Scott. L. Collins A1 - Fahey, T.J. A1 - Hansen, G.J.A. A1 - Landis, D.A. A1 - Kimberly J. La Pierre A1 - Melillo, J.M. A1 - Seastedt, T.R. A1 - Shaver, G.R. A1 - Webster, J.R. KW - Climate change KW - global change KW - long-term research KW - LTER Network KW - multifactor experiments AB -

The US National Science Foundation–funded Long Term Ecological Research (LTER) Network supports a large (around 240) and diverse portfolio of long-term ecological experiments. Collectively, these long-term experiments have (a) provided unique insights into ecological patterns and processes, although such insight often became apparent only after many years of study; (b) influenced management and policy decisions; and (c) evolved into research platforms supporting studies and involving investigators who were not part of the original design. Furthermore, this suite of long-term experiments addresses, at the site level, all of the US National Research Council's Grand Challenges in Environmental Sciences. Despite these contributions, we argue that the scale and scope of global environmental change requires a more-coordinated multisite approach to long-term experiments. Ideally, such an approach would include a network of spatially extensive multifactor experiments, designed in collaboration with ecological modelers that would build on and extend the unique context provided by the LTER Network.

VL - 62 UR - https://academic.oup.com/bioscience/article/62/4/377/243762 ER - TY - JOUR T1 - A test of two mechanisms proposed to optimize grassland aboveground primary productivity in response to grazing JF - Journal of Plant Ecology Y1 - 2012 A1 - Alan K. Knapp A1 - D.L. Hoover A1 - John M. Blair A1 - Buis, G. A1 - Burkepile, D.E. A1 - Chamberlain, A.J. A1 - Scott. L. Collins A1 - Fynn, R.W.S. A1 - Kirkman, K.P. A1 - M.D. Smith A1 - Blake, D. A1 - Govender , N. A1 - O’Neal, P. A1 - Schreck, T. A1 - Zinn, A. KW - aboveground net primary production KW - fire KW - forbs KW - herbivores KW - savanna AB -

Aims Mesic grasslands have a long evolutionary history of grazing by large herbivores and as a consequence, grassland species have numerous adaptations allowing them to respond favourably to grazing. Although empirical evidence has been equivocal, theory predicts that such adaptations combined with alterations in resources can lead to grazing-induced overcompensation in aboveground net primary production (ANPP; grazed ANPP > ungrazed ANPP) under certain conditions. We tested two specific predictions from theory. First, overcompensation is more likely to occur in annually burned grasslands because limiting nutrients that would be lost with frequent fires are recycled through grazers and stimulate ANPP. Second, overcompensation of biomass lost to grazers is more likely to occur in unburned sites where grazing has the greatest effect on increasing light availability through alterations in canopy structure. Methods We tested these nutrient versus light-based predictions in grazed grasslands that had been annually burned or protected from fire for >20 years. We assessed responses in ANPP to grazing by large ungulates using both permanent and moveable grazing exclosures (252 exclosures from which biomass was harvested from 3192 quadrats) in a 2-year study. Study sites were located at the Konza Prairie Biological Station (KPBS) in North America and at Kruger National Park (KNP) in South Africa. At KPBS, sites were grazed by North American bison whereas in KNP sites were grazed either by a diverse suite of herbivores (e.g. blue wildebeest, Burchell’s zebra, African buffalo) or by a single large ungulate (African buffalo). Important Findings We found no evidence for overcompensation in either burned or unburned sites, regardless of grazer type. Thus, there was no support for either mechanism leading to overcompensation. Instead, complete compensation of total biomass lost to grazers was the most common response characterizing grazing–ANPP relationships with, in some cases, undercompensation of grass ANPP being offset by increased ANPP of forbs likely due to competitive release. The capability of these very different grass-dominated systems to maintain ANPP while being grazed has important implications for energy flow, ecosystem function and the trophic dynamics of grasslands.

VL - 5 UR - https://academic.oup.com/jpe/article/5/4/357/908695 ER - TY - JOUR T1 - Assessing fine-scale genotypic structure of a dominant species in native grasslands JF - The American Midland Naturalist Y1 - 2011 A1 - M.L. Avolio A1 - Chang, C.C. A1 - M.D. Smith AB -

Genotypic diversity of dominant species has been shown to have important consequences for community and ecosystem processes at a fine spatial scale. We examined the fine-scale (i.e., plant neighborhood scale, <1 m2) genotypic structure of Andropogon gerardii, a dominant species in the tallgrass prairie, which is a productive and endangered grassland ecosystem, employing the commonly used amplified fragment length polymorphism (AFLP) technique. In this paper we used two methods to assess the fine-scale genetic spatial structure of a dominant perennial grass, (1) we determined how many tillers to sample in a 1 m2 area and (2) we developed AFLP markers that would differentiate between genotypes. By determining appropriate sampling and molecular techniques, our findings can be applied to questions addressing how genetic diversity of dominant species affect ecosystem processes in the tallgrass prairie.

VL - 165 UR - https://doi.org/10.1674/0003-0031-165.2.211 ER - TY - JOUR T1 - Explaining temporal variation in above-ground productivity in a mesic grassland: the role of climate and flowering JF - Journal of Ecology Y1 - 2011 A1 - Kimberly J. La Pierre A1 - Yuan, S.H. A1 - Chang, C.C. A1 - M.L. Avolio A1 - Hallett, L.M. A1 - Schreck, T. A1 - M.D. Smith AB -

1. Annual above-ground net primary productivity (ANPP) in mesic grasslands is known to be highly temporally variable. While yearly precipitation or average yearly temperature can explain some of this temporal variability, much of the variation in ANPP remains unexplained. 2. Here we address the heretofore unexplained variation in 25 years of productivity data from a mesic grassland at Konza Prairie (north-eastern Kansas) by examining the effects of precipitation and temperature during periods relevant to the phenology and growth cycle of the dominant C4 grasses and the flowering stalk production of these species. We assessed both the direct effects and indirect effects via flowering of phenologically relevant climate periods on ANPP using structural equation modelling (SEM). 3. We found ANPP to be strongly positively influenced by flowering stalk production of the dominant C4 grasses, precipitation during periods relevant to vegetative growth (15 April–14 July) and flowering stalk elongation (15 July–14 August) of the dominant grasses, and fire. In addition, flowering stalk production was negatively influenced by high temperatures during the flowering stalk elongation period, which therefore resulted in a negative indirect effect on ANPP. We found little evidence for the effects of the previous year’s total annual precipitation or mean annual temperature on ANPP. 4. By including flowering stalk production and separating climate variables into phenologically relevant periods we were able to increase the percentage of observed variance in ANPP explained by six models, relating to different topographic positions and burn regimes, from an average of 22% to 48%, with the best model explaining 61% of variation in ANPP. 5.Synthesis. The link between climatic periods relevant to the phenology and growth of dominant C4 grasses, flowering stalk production of these grasses and ANPP shown here improves our ability to predict productivity in mesic grasslands, an ecologically and economically important ecosystem.

VL - 99 UR - https://besjournals.onlinelibrary.wiley.com/doi/full/10.1111/j.1365-2745.2011.01844.x ER - TY - JOUR T1 - An integrated conceptual framework for long-term social-ecological research JF - Frontiers in Ecology and the Environment Y1 - 2011 A1 - Scott. L. Collins A1 - Carpenter, S.R. A1 - Swinton, S.M. A1 - Orenstein, D.E. A1 - Childers, D.L. A1 - Gragson, T.L. A1 - Grimm, N.B. A1 - Grove, J.M. A1 - Harlan, S.L. A1 - Kaye, J.P. A1 - Alan K. Knapp A1 - Kofinas, G.P. A1 - Magnuson, J.J. A1 - W.H. McDowell A1 - Melack, J.M. A1 - Ogden, L.A. A1 - Robertson, G.R. A1 - M.D. Smith A1 - Whitmer, A.C. AB -

The global reach of human activities affects all natural ecosystems, so that the environment is best viewed as a social–ecological system. Consequently, a more integrative approach to environmental science, one that bridges the biophysical and social domains, is sorely needed. Although models and frameworks for social–ecological systems exist, few are explicitly designed to guide a long-term interdisciplinary research program. Here, we present an iterative framework, “Press–Pulse Dynamics” (PPD), that integrates the biophysical and social sciences through an understanding of how human behaviors affect “press” and “pulse” dynamics and ecosystem processes. Such dynamics and processes, in turn, influence ecosystem services –thereby altering human behaviors and initiating feedbacks that impact the original dynamics and processes. We believe that research guided by the PPD framework will lead to a more thorough understanding of social–ecological systems and generate the knowledge needed to address pervasive environmental problems.

VL - 9 UR - https://esajournals.onlinelibrary.wiley.com/doi/abs/10.1890/100068 ER - TY - JOUR T1 - Relative effects of precipitation variability and warming on tallgrass prairie ecosystem function JF - Biogeosciences Y1 - 2011 A1 - Fay, P.A. A1 - John M. Blair A1 - M.D. Smith A1 - Jesse B. Nippert A1 - Carlisle, J.D. A1 - Alan K. Knapp AB -

Precipitation and temperature drive many aspects of terrestrial ecosystem function. Climate change scenarios predict increasing precipitation variability and temperature, and long term experiments are required to evaluate the ecosystem consequences of interannual climate variation, increased growing season (intra-annual) rainfall variability, and warming. We present results from an experiment applying increased growing season rainfall variability and year round warming in native tallgrass prairie. During ten years of study, total growing season rainfall varied 2-fold, and we found ~50–200% interannual variability in plant growth and aboveground net primary productivity (ANPP), leaf carbon assimilation (ACO2), and soil CO2 efflux (JCO2) despite only ~40% variation in mean volumetric soil water content (0–15 cm, Θ15). Interannual variation in soil moisture was thus amplified in most measures of ecosystem response. Differences between years in Θ15 explained the greatest portion (14–52%) of the variation in these processes. Experimentally increased intra-annual season rainfall variability doubled the amplitude of intra-annual soil moisture variation and reduced Θ15 by 15%, causing most ecosystem processes to decrease 8–40% in some or all years with increased rainfall variability compared to ambient rainfall timing, suggesting reduced ecosystem rainfall use efficiency. Warming treatments increased soil temperature at 5 cm depth, particularly during spring, fall, and winter. Warming advanced canopy green up in spring, increased winter JCO2, and reduced summer JCO2 and forb ANPP, suggesting that the effects of warming differed in cooler versus warmer parts of the year. We conclude that (1) major ecosystem processes in this grassland may be substantially altered by predicted changes in interannual climate variability, intra-annual rainfall variability, and temperature, (2) interannual climate variation was a larger source of variation in ecosystem function than intra-annual rainfall variability and warming, and (3) effects of increased growing season rainfall variability and warming were small, but ecologically important. The relative effects of these climate drivers are likely to vary for different ecosystem processes and in wetter or drier ecosystems.

VL - 8 UR - https://www.biogeosciences.net/8/3053/2011/ ER - TY - JOUR T1 - Fire and grazing impacts on silica production and storage in grass dominated ecosystems JF - Biogeochemistry Y1 - 2010 A1 - Melzer, S.E. A1 - Alan K. Knapp A1 - Fynn, R.W.S. A1 - Kirkman, K.P. A1 - M.D. Smith A1 - John M. Blair A1 - Kelly, E.F. KW - Biogenic silica KW - North American grasslands KW - Soil South African savannas KW - Terrestrial plants AB -

Grassland ecosystems are an important terrestrial component of the global biogeochemical silicon cycle. Although the structure and ecological functioning of grasslands are strongly influenced by fire and grazing, the role of these key ecological drivers in the production and storage of silicon represents a significant knowledge gap, particularly since they are being altered worldwide by human activities. We evaluated the effects of fire and grazing on the range and variability of plant derived biogenic silica stored in plant biomass and soils by sampling plants and soils from long-term experimental plots with known fire and grazing histories. Overall, plants and soils from grazed sites in the South African ecosystems had up to 76 and 54% greater biogenic silica totals (kg ha−1), respectively, than grazed North American sites. In North American soils, the combination of grazing and annual fire resulted in the greatest abundance of biogenic silica, whereas South African soils had the highest biogenic silica content where grazed regardless of burn frequency. These results as well as those that show greater Si concentrations in grazed South African plants indicate that South African plants and soils responded somewhat differently to fire and grazing with respect to silicon cycling, which may be linked to differences in the evolutionary history and in the grazer diversity and grazing intensity of these ecosystems. We conclude that although fire and grazing (as interactive and/or independent factors) do not affect the concentration of Si taken up by plants, they do promote increased silicon storage in aboveground biomass and soil as a result of directly affecting other site factors such as aboveground net primary productivity. Therefore, as management practices, fire and grazing have important implications for assessing global change impacts on the terrestrial biogeochemical cycling of silicon.

VL - 97 UR - https://link.springer.com/article/10.1007%2Fs10533-009-9371-3 ER - TY - JOUR T1 - Variation in gene expression of Andropogon gerardii in response to altered environmental conditions associated with climate change JF - Journal of Ecology Y1 - 2010 A1 - Travers, S.E. A1 - Tang, Z. A1 - Caragea, D. A1 - Garrett, K.A. A1 - Hulbert, S.H. A1 - Leach, J.E. A1 - Bai, J. A1 - Saleh, A. A1 - Alan K. Knapp A1 - Fay, P.A. A1 - Jesse B. Nippert A1 - Schnable, P.S. A1 - M.D. Smith AB -

1. If we are to understand the mechanisms underlying species responses to climate change in natural systems, studies are needed that focus on responses of non-model species under field conditions. We measured transcriptional profiles of individuals of Andropogon gerardii, a C4 grass native to North American grasslands, in a field experiment in which both temperature and precipitation were manipulated to simulate key aspects of forecasted climate change. 2. By using microarrays developed for a closely related model species, Zea mays, we were able to compare the relative influence of warming versus altered soil moisture availability on expression levels of over 7000 genes, identify responsive functional groups of genes and correlate changes in gene transcription with physiological responses. 3. We observed more statistically significant shifts in transcription levels of genes in response to thermal stress than in response to water stress. We also identified candidate genes that demonstrated transcription levels closely associated with physiological variables, in particular chlorophyll fluorescence. 4.Synthesis. These results suggest that an ecologically important species responds differently to different environmental aspects of forecast climate change. These translational changes have the potential to influence phenotypic characters and ultimately adaptive responses.

VL - 98 UR - https://besjournals.onlinelibrary.wiley.com/doi/full/10.1111/j.1365-2745.2009.01618.x ER - TY - JOUR T1 - Controls of aboveground net primary production in mesic savanna grasslands: An inter-hemispheric comparison JF - Ecosystems Y1 - 2009 A1 - Buis, G.M. A1 - John M. Blair A1 - Burkepile, D.E. A1 - Burns, C.E. A1 - Chamberlain, A.J. A1 - Chapman, P. A1 - Scott. L. Collins A1 - Fynn, R.W.S. A1 - Govender, N. A1 - Kirkman, K. A1 - M.D. Smith A1 - Alan K. Knapp KW - ANPP KW - fire KW - Grasslands KW - Grazing KW - nitrogen KW - Savannas AB -

Patterns and controls of annual aboveground net primary productivity (ANPP) are fundamental metrics of ecosystem functioning. It is generally assumed, but rarely tested, that determinants of ANPP in one region within a biome will operate similarly throughout that biome, as long as physiognomy and climate are broadly consistent. We tested this assumption by quantifying ANPP responses to fire, grazing history, and nitrogen (N) addition in North American (NA) and South African (SA) savanna grasslands. We found that total ANPP responded in generally consistent ways to fire, grazing history, and N addition on both continents. Annual fire in both NA and SA consistently stimulated total ANPP (28–100%) relative to unburned treatments at sites with deep soils, and had no effect on ANPP in sites with shallow soils. Fire did not affect total ANPP in sites with a recent history of grazing, regardless of whether a single or a diverse suite of large herbivores was present. N addition interacted strongly and consistently with fire regime in both NA and SA. In annually burned sites that were not grazed, total ANPP was stimulated by N addition (29–39%), but there was no effect of N fertilization in the absence of fire. In contrast, responses in forb ANPP to fire and grazing were somewhat divergent across this biome. Annual fire in NA reduced forb ANPP, whereas grazing increased forb ANPP, but neither response was evident in SA. Thus, despite a consistent response in total ANPP, divergent responses in forb ANPP suggest that other aspects of community structure and ecosystem functioning differ in important ways between these mesic savanna grasslands.

VL - 12 UR - https://link.springer.com/article/10.1007%2Fs10021-009-9273-1 ER - TY - JOUR T1 - Ecophysiological responses of two dominant grasses to altered temperature and precipitation regimes JF - Acta Oecologia Y1 - 2009 A1 - Jesse B. Nippert A1 - Fay, P.A. A1 - Carlisle, J.D. A1 - Alan K. Knapp A1 - M.D. Smith KW - Andropogon gerardii KW - Climate variability KW - Leaf gas exchange KW - RaMPs KW - Sensitivity KW - Sorghastrum nutans AB -

Ecosystem responses to climate change will largely be driven by responses of the dominant species. However, if co-dominant species have traits that lead them to differential responses, then predicting how ecosystem structure and function will be altered is more challenging. We assessed differences in response to climate change factors for the two dominant C4 grass species in tallgrass prairie, Andropogon gerardii and Sorghastrum nutans, by measuring changes in a suite of plant ecophysiological traits in response to experimentally elevated air temperatures and increased precipitation variability over two growing seasons. Maximum photosynthetic rates, stomatal conductance, water-use efficiency, chlorophyll fluorescence, and leaf water potential varied with leaf and canopy temperature as well as with volumetric soil water content (0–15 cm). Both species had similar responses to imposed changes in temperature and water availability, but when differences occurred, responses by A. gerardii were more closely linked with changes in air temperature whereas S. nutans was more sensitive to changes in water availability. Moreover, S. nutans was more responsive overall than A. gerardii to climate alterations. These results indicate both grass species are responsive to forecast changes in temperature and precipitation, but their differential sensitivity to temperature and water availability suggest that future population and community structure may vary based on the magnitude and scope of an altered climate.

VL - 35 UR - https://www.sciencedirect.com/science/article/pii/S1146609X09000204?via%3Dihub ER - TY - JOUR T1 - A framework for assessing ecosystem dynamics in response to chronic resource alterations induced by global change JF - Ecology Y1 - 2009 A1 - M.D. Smith A1 - Alan K. Knapp A1 - Scott. L. Collins AB -

In contrast to pulses in resource availability following disturbance events, many of the most pressing global changes, such as elevated atmospheric carbon dioxide concentrations and nitrogen deposition, lead to chronic and often cumulative alterations in available resources. Therefore, predicting ecological responses to these chronic resource alterations will require the modification of existing disturbance-based frameworks. Here, we present a conceptual framework for assessing the nature and pace of ecological change under chronic resource alterations. The “hierarchical-response framework” (HRF) links well-documented, ecological mechanisms of change to provide a theoretical basis for testing hypotheses to explain the dynamics and differential sensitivity of ecosystems to chronic resource alterations. The HRF is based on a temporal hierarchy of mechanisms and responses beginning with individual (physiological/metabolic) responses, followed by species reordering within communities, and finally species loss and immigration. Each mechanism is hypothesized to differ in the magnitude and rate of its effects on ecosystem structure and function, with this variation depending on ecosystem attributes, such as longevity of dominant species, rates of biogeochemical cycling, levels of biodiversity, and trophic complexity. Overall, the HRF predicts nonlinear changes in ecosystem dynamics, with the expectation that interactions with natural disturbances and other global-change drivers will further alter the nature and pace of change. The HRF is explicitly comparative to better understand differential sensitivities of ecosystems, and it can be used to guide the design of coordinated, cross-site experiments to enable more robust forecasts of contemporary and future ecosystem dynamics.

VL - 90 UR - https://esajournals.onlinelibrary.wiley.com/doi/abs/10.1890/08-1815.1 ER - TY - JOUR T1 - Comparison of damage to native and exotic tallgrass prairie plants by natural enemies JF - Plant Ecology Y1 - 2008 A1 - Han, X. A1 - Dend, S.P. A1 - Garrett, K.A. A1 - Fang, L. A1 - M.D. Smith KW - Enemy release hypothesis KW - grassland KW - Invasiveness KW - Leaf damage KW - Rust fungi AB -

We surveyed the prevalence and amount of leaf damage related to herbivory and pathogens on 12 pairs of exotic (invasive and noninvasive) and ecologically similar native plant species in tallgrass prairie to examine whether patterns of damage match predictions from the enemy release hypothesis. We also assessed whether natural enemy impacts differed in response to key environmental factors in tallgrass prairie by surveying the prevalence of rust on the dominant C4 grass, Andropogon gerardii, and its congeneric invasive exotic C4 grass, A. bladhii, in response to fire and nitrogen fertilization treatments. Overall, we found that the native species sustain 56.4% more overall leaf damage and 83.6% more herbivore-related leaf damage when compared to the exotic species. Moreover, we found that the invasive exotic species sustained less damage from enemies relative to their corresponding native species than the noninvasive exotic species. Finally, we found that burning and nitrogen fertilization both significantly increased the prevalence of rust fungi in the native grass, while rust fungi rarely occurred on the exotic grass. These results indicate that reduced damage from enemies may in part explain the successful naturalization of exotic species and the spread of invasive exotic species in tallgrass prairie.

VL - 198 UR - https://link.springer.com/article/10.1007%2Fs11258-008-9395-0 ER - TY - JOUR T1 - Consequences of more extreme precipitation regimes for terrestrial ecosystems JF - BioScience Y1 - 2008 A1 - Alan K. Knapp A1 - Beier, C. A1 - Briske, D.D. A1 - Classen, A.T. A1 - Luo, Y. A1 - Reichstein, M. A1 - M.D. Smith A1 - Smith, S.D. A1 - Bell, J.E. A1 - Fay, P.A. A1 - Heisler, J.L. A1 - Leavitt, S.W A1 - Sherry, R. A1 - Smith, B. A1 - Weng, E. KW - Climate change KW - Drought KW - Ecosystems KW - Precipitation KW - soil water AB -

mplification of the hydrological cycle as a consequence of global warming is forecast to lead to more extreme intra-annual precipitation regimes characterized by larger rainfall events and longer intervals between events. We present a conceptual framework, based on past investigations and ecological theory, for predicting the consequences of this underappreciated aspect of climate change. We consider a broad range of terrestrial ecosystems that vary in their overall water balance. More extreme rainfall regimes are expected to increase the duration and severity of soil water stress in mesic ecosystems as intervals between rainfall events increase. In contrast, xeric ecosystems may exhibit the opposite response to extreme events. Larger but less frequent rainfall events may result in proportional reductions in evaporative losses in xeric systems, and thus may lead to greater soil water availability. Hydric (wetland) ecosystems are predicted to experience reduced periods of anoxia in response to prolonged intervals between rainfall events. Understanding these contingent effects of ecosystem water balance is necessary for predicting how more extreme precipitation regimes will modify ecosystem processes and alter interactions with related global change drivers.

VL - 58 UR - https://academic.oup.com/bioscience/article/58/9/811/250853 ER - TY - CHAP T1 - Ecological consequences of the replacement of native grassland by Juniperus virginiana and other woody plants T2 - Ecological Studies Vol. 196, Western North American Juniperus communities: A dynamic vegetation type Y1 - 2008 A1 - Alan K. Knapp A1 - McCarron, J.K. A1 - Silletti, A.M. A1 - Hoch, G.A. A1 - Heisler, J.L. A1 - Lett, M.S. A1 - John M. Blair A1 - J. M. Briggs A1 - M.D. Smith ED - Van Auken, O.W. AB -

Although grasslands have been altered by humans for thousands of years (Wedel 1961; Bond et al. 2003), the loss of grassland as a result of anthropogenic activities has increased dramatically over the past 150 years. When Europeans first settled the Midwest and Great Plains, the greatest threat to native grasslands was the conversion of the most highly productive of these ecosystems to row-crop agriculture (Samson and Knopf 1994). Later, with improvements in soil moisture management and irrigation technology, even low-productivity grasslands were plowed. Today, those remnants of the most productive grasslands that escaped the plow are threatened, as are most of Earth’s ecosystems, by a variety of global change phenomena (Vitousek et al. 1997), with the invasion and expansion of woody species into grasslands one of the greatest of these threats. The replacement of grasslands by shrubland, woodland, and forest is a concern not only in the United States but worldwide (Archer et al. 1988; Van Auken 2000; Roques et al. 2001; Silva et al. 2001). Species of woody plants that invade grasslands may include both native plants which previously existed as more minor components of the ecosystem as well as alien species (Bragg and Hulbert 1976; Harcombe et al. 1993).

JF - Ecological Studies Vol. 196, Western North American Juniperus communities: A dynamic vegetation type PB - Springer-Verlag, NY UR - https://link.springer.com/chapter/10.1007%2F978-0-387-34003-6_8 ER - TY - JOUR T1 - Influence of grazing and fire frequency on small-scale plant community structure and resource variability in native tallgrass prairie JF - Oikos Y1 - 2008 A1 - Veen, G.F. A1 - John M. Blair A1 - M.D. Smith A1 - Scott. L. Collins AB -

In grasslands worldwide, grazing by ungulates and periodic fires are important forces affecting resource availability and plant community structure. It is not clear, however, whether changes in community structure are the direct effects of the disturbance (i.e. fire and grazing) or are mediated indirectly through changes in resource abundance and availability. In North American tallgrass prairies, fire and grazing often have disparate effects on plant resources and plant diversity, yet, little is known about the individual and interactive effects of fire and grazing on resource variability and how that variability relates to heterogeneity in plant community structure, particularly at small scales. We conducted a field study to determine the interactive effects of different long-term fire regimes (annual vs four-year fire frequency) and grazing by native ungulates (Bos bison) on small-scale plant community structure and resource variability (N and light) in native tallgrass prairie. Grazing enhanced light and nitrogen availability, but did not affect small-scale resource variability. In addition, grazing reduced the dominance of C4 grasses which enhanced species richness, diversity and community heterogeneity. In contrast, annual fire increased community dominance and reduced species richness and diversity, particularly in the absence of grazing, but had no effect on community heterogeneity, resource availability and resource variability. Variability in the abundance of resources showed no relationship with community heterogeneity at the scale measured in this study, however we found a relationship between community dominance and heterogeneity. Therefore, we conclude that grazing generated small-scale community heterogeneity in this mesic grassland by directly affecting plant community dominance, rather than indirectly through changes in resource variability.

VL - 117 UR - https://onlinelibrary.wiley.com/doi/abs/10.1111/j.0030-1299.2008.16515.x ER - TY - JOUR T1 - Does species diversity limitproductivity in natural grassland communities? JF - Ecology Letters Y1 - 2007 A1 - Grace, J.B. A1 - Anderson, T.M. A1 - M.D. Smith A1 - Seabloom, E. A1 - Andelman, S.J. A1 - Meche, G. A1 - Weiher, E. A1 - Allain, L.K. A1 - Jutila, H. A1 - Sankaran, M. A1 - Knops, J. A1 - Ritchie, M. A1 - M.R. Whiles AB - Theoretical analyses and experimental studies of synthesized assemblages indicate that under particular circumstances species diversity can enhance community productivity through niche complementarity. It remains unclear whether this process has important effects in mature natural ecosystems where competitive feedbacks and complex environmental influences affect diversity–productivity relationships. In this study, we evaluated diversity–productivity relationships while statistically controlling for environmental influences in 12 natural grassland ecosystems. Because diversity–productivity relationships are conspicuously nonlinear, we developed a nonlinear structural equation modeling (SEM) methodology to separate the effects of diversity on productivity from the effects of productivity on diversity. Meta-analysis was used to summarize the SEM findings across studies. While competitive effects were readily detected, enhancement of production by diversity was not. These results suggest that the influence of small-scale diversity on productivity in mature natural systems is a weak force, both in absolute terms and relative to the effects of other controls on productivity. VL - 10 ER - TY - JOUR T1 - Ecological genomics: making the leap from model systems in the lab to native populations in the field. Frontiers in Ecology and the Environment JF - Frontiers in Ecology and the Environment Y1 - 2007 A1 - Travers, S.E. A1 - M.D. Smith A1 - Bai, J. A1 - Hulbert, S.H. A1 - Leach, J.E. A1 - Schnable, P.S. A1 - Alan K. Knapp A1 - Milliken, G.A. A1 - Fay, P.A. A1 - Saleh, A. A1 - Garrett, K.A. AB - Recent reviews have emphasized the need to incorporate genomics into ecological field studies to further understand how species respond to changing environmental conditions. Genomic tools, such as cDNA (complementary DNA) microarrays, allow for the simultaneous analysis of gene expression of thousands of genes from all or part of an organism's genome (the transcription profile), thereby revealing the genetic mechanisms that underlie species' responses to environmental change. However, despite their potential, two major limitations have hindered the incorporation of microarrays and other genomic tools into field studies: (1) the limited availability of microarrays for ecologically relevant, non-model species and limited financial resources for developing new microarrays; and (2) concern that high sensitivity of gene expression to even subtle alterations in environmental conditions will hinder detection of relevant changes in field measures of transcription profiles. Here, we show that with cross-species hybridizations of microarrays developed for a closely related model organism, an appropriate experimental design, and sufficient replication, transcriptional profiling can successfully be incorporated into field studies. In this way, relevant changes in gene expression with changing environmental conditions can be detected. VL - 5 ER - TY - JOUR T1 - Convergence and contingency in production-precipitation relationships in North American and South African C4 grasslands JF - Oecologia Y1 - 2006 A1 - Alan K. Knapp A1 - Burns, C.E. A1 - Fynn, R.W.S. A1 - Kirkman, K.P. A1 - Morris, C.D. A1 - M.D. Smith KW - climate KW - Evolutionary history KW - Net primary production KW - Precipitation KW - soil moisture AB - Mesic grasslands in North America and South Africa share many structural attributes, but less is known of their functional similarities. We assessed the control of a key ecosystem process, aboveground net primary production (ANPP), by interannual variation in precipitation amount and pattern via analysis of data sets (15- and 24-year periods) from long-term research programs on each continent. Both sites were dominated by C4 grasses and had similar growing season climates; thus, we expected convergence in precipitation–ANPP relationships. Lack of convergence, however, would support an alternative hypothesis—that differences in evolutionary history and purportedly greater climatic variability in South Africa fundamentally alter the functioning of southern versus northern hemisphere grasslands. Neither mean annual precipitation nor mean ANPP differed between the South African and North American sites (838 vs. 857 mm/year, 423.5 vs. 461.4 g/m2 respectively) and growing season precipitation–ANPP relationships were similar. Despite overall convergence, there were differences between sites in how the seasonal timing of precipitation affected ANPP. In particular, interannual variability in precipitation that fell during the first half of the growing season strongly affected annual ANPP in South Africa (P < 0.01), but was not related to ANPP in North America (P = 0.098). Both sites were affected similarly by late season precipitation. Divergence in the seasonal course of available soil moisture (chronically low in the winter and early spring in the South African site vs. high in the North American site) is proposed as a key contingent factor explaining differential sensitivity in ANPP to early season precipitation in these two grasslands. These long-term data sets provided no support for greater rainfall, temperature or ANPP variability in the South African versus the North American site. However, greater sensitivity of ANPP to early season precipitation in the South African grassland suggests that future patterns of productivity may be more responsive to seasonal changes in climate compared with the North American site. VL - 149 ER - TY - JOUR T1 - Growth responses of twodominant C4 grass species to altered water availability JF - InternationalJournal of Plant Sciences Y1 - 2006 A1 - Swemmer, A.M. A1 - Alan K. Knapp A1 - M.D. Smith KW - Andropogon gerardii KW - biomass allocation KW - Climate change KW - leaf turnover KW - photosynthesis KW - Sorghastrum nutans KW - water stress AB - Identifying key ecophysiological traits that differ among dominant plant species and can be linked to species‐specific responses to drought would improve our ability to forecast community and ecosystem responses to global climate change. The mesic grasslands of the central plains of North America are dominated by two C4 grass species, Andropogon gerardii and Sorghastrum nutans, which purportedly differ in their tolerance of water stress. Individuals of these two species were grown in the field under rain‐out shelters and subjected to wet (watered every 2–3 d) or dry (repeatedly subjected to wilting before watering) soil moisture regimes. A range of ecophysiological traits potentially important for tolerating water stress were concurrently measured. Although few traits differed between the species in the wet treatment, several traits were identified in the dry treatment that may enable A. gerardii to better tolerate drought. These were greater allocation to roots, reduced allocation to flowering, more rapid leaf turnover, and more rapid recovery of photosynthesis after wilting. The latter two traits may be particularly important for coping with increased variability in rainfall regimes in the future and are consistent with recently documented responses of A. gerardii to experimental increases in soil moisture variability. VL - 167 ER - TY - JOUR T1 - Scale-dependent interaction of fire andgrazing on community heterogeneity in tallgrass prairie JF - Ecology Y1 - 2006 A1 - Scott. L. Collins A1 - M.D. Smith AB - Natural disturbances affect spatial and temporal heterogeneity in plant communities, but effects vary depending on type of disturbance and scale of analysis. In this study, we examined the effects of fire frequency (1-, 4-, and 20-yr intervals) and grazing by bison on spatial and temporal heterogeneity in species composition in tallgrass prairie plant communities. Compositional heterogeneity was estimated at 10-, 50-, and 200-m2 scales. For each measurement scale, we used the average Euclidean Distance (ED) between samples within a year (2000) to measure spatial heterogeneity and between all time steps (1993–2000) for each sample to measure temporal heterogeneity. The main effects of fire and grazing were scale independent. Spatial and temporal heterogeneity were lowest on annually burned sites and highest on infrequently burned (20-yr) sites at all scales. Grazing reduced spatial heterogeneity and increased temporal heterogeneity at all scales. The rate of community change over time decreased as fire frequency increased at all scales, whereas grazing had no effect on rate of community change over time at any spatial scale. The interactive effects of fire and grazing on spatial and temporal heterogeneity differed with scale. At the 10-m2 scale, grazing increased spatial heterogeneity in annually burned grassland but decreased heterogeneity in less frequently burned areas. At the 50-m2 scale, grazing decreased spatial heterogeneity on 4-yr burns but had no effect at other fire frequencies. At the 10-m2 scale, grazing increased temporal heterogeneity only on 1- and 20-yr burn sites. Our results show that the individual effects of fire and grazing on spatial and temporal heterogeneity in mesic prairie are scale independent, but the interactive effects of these disturbances on community heterogeneity change with scale of measurement. These patterns reflect the homogenizing impact of fire at all spatial scales, and the different frequency, intensity, and scale of patch grazing by bison in frequently burned vs. infrequently burned areas. VL - 87 ER - TY - JOUR T1 - Indicators of plant species richness in AVIRIS spectra of a mesic grassland JF - Remote Sensing of Environment Y1 - 2005 A1 - Carter, G.A. A1 - Alan K. Knapp A1 - Anderson, J.E. A1 - Hoch, G.A. A1 - M.D. Smith KW - AVIRIS KW - Biodiversity KW - Bison grazing KW - grassland KW - Hyperspectral KW - Plant species richness KW - Radiance KW - Reflectance KW - Soil exposure KW - Spatial variability KW - Spectral mixing AB - Hyperspectral imagery of the Konza Prairie Biological Station in northeastern Kansas was used to evaluate upwelling spectral radiance, prairie spectral reflectance and band ratios of each as potential indicators of vascular plant species richness in a mesic grassland. The extent to which spatial variability in these parameters related to plant species richness also was investigated. A 224 channel hyperspectral data cube acquired in June 2000 by the Airborne Visible and Infrared Imaging Spectrometer (AVIRIS) provided complete coverage of the 400–2500 nm range at approximately 10 nm per channel. After band deletions accounted for detector overlap and strong atmospheric attenuation features, 176 bands were retained for analysis and spanned the 404–2400 nm range. Prairie reflectance was estimated via radiative transfer modeling and scaling to a library spectrum of highway construction material. Data were sampled from pixels having a 19 m ground sample distance (GSD) to represent each of 93 vegetation sampling transects. Reflectance and radiance at mid-infrared wavelengths (e.g., 1553 nm), and band ratios that were based on atmospheric windows in the red, near-infrared and mid-infrared spectra estimated species richness to within 6 to 7 species per transect. The 856 to 780 nm radiance or reflectance ratio yielded maximum adjusted coefficients of determination (r2) of approximately 0.4 in regressions with richness when data from bison-grazed and ungrazed areas were combined. These regressions remained significant (p ≤ 0.001) when only ungrazed areas were assessed although r2 reduced to approximately 0.2. Richness was related significantly also to the 433 to 674 nm reflectance ratio for grazed-plus-ungrazed and ungrazed-only areas. In contrast, the effectiveness of the 433 to 674 nm radiance ratio was reduced by atmospheric backscatter. Species richness did not correlate strongly or consistently with transect spatial variability (coefficient of variation or range) in radiance, reflectance or band ratio value, apparently as a consequence of the relatively small area sampled for each transect (approximately 0.5 ha). Relationships between richness and prairie spectral features were explained by the influence of soil exposure on both parameters. Richness and estimated soil exposure tended to increase from ungrazed lowlands, to ungrazed slopes, to ungrazed uplands to grazed areas. Remotely sensed estimates of soil exposure may be particularly useful in addressing plant species richness on grazed grasslands owing to an overall similarity in spectral reflectance among dominant plant species. VL - 98 ER - TY - JOUR T1 - A test for community change using a null model approach JF - Ecological Applications Y1 - 2005 A1 - Schaefer, J. A1 - K. B. Gido A1 - M.D. Smith AB - Quantifying patterns of temporal or spatial change in community structure is critical for assessing the impact of disturbances on biological systems and the stability of ecosystems. Detecting change in communities can be problematic, however, because of the inherent variability of systems and limitations of commonly used methods, such as similarity indices and ordination that do not explicitly test a hypothesis. Here we present empirical data to show a strong relationship between species mean abundance and variability across three broad taxonomic groups (plants, zooplankton, and fish). These statistical relationships were then used to construct null models of expected community variability that were used to test against the observed temporal change of these communities. We evaluated the ability of this approach to detect significant temporal change above that associated with random variation with nine communities (three Midwestern stream fish, three north temperate zooplankton, and three tallgrass prairie plant), each having long-term data sets and different expected levels of change. Nonrandom change was detected in 21.3% of samples from the expected low-change communities, 52.6% in moderately changed communities, and 60.4% in high-change communities. Thus, this approach was effective in detecting change over time in those communities expected to change most. By using empirical relationships between species abundance and variability, this null model approach provides ecologists and resource managers an objective tool, which can be used along with existing community indices and statistical techniques to assess the type and magnitude of community change with limited data sets. VL - 15 ER - TY - JOUR T1 - Convergence across biomes to a common rain-use efficiency JF - Nature Y1 - 2004 A1 - Huxman, T.E. A1 - M.D. Smith A1 - Fay, P.A. A1 - Alan K. Knapp A1 - Shaw, M.R. A1 - Loik, M.E. A1 - Smith, S.D. A1 - Tissue, D.T. A1 - Zak, J.C. A1 - Weltzin, J.F. A1 - Pockman, W.T. A1 - Sala, O.E. A1 - Haddad, B.M. A1 - Harte, J. A1 - Koch, G.W. A1 - Schwinning, S. A1 - Small, E.E. A1 - Williams, D.G. VL - 429 ER - TY - JOUR T1 - Dominance not diversity determines invasibility of tallgrass prairie JF - Oikos Y1 - 2004 A1 - M.D. Smith A1 - Wilcox, J. A1 - Kelly, T. A1 - Alan K. Knapp AB - Many recent studies suggest that more diverse communities are more resistant to invasion. Community characteristics that most strongly influence invasion are uncertain, however, due to covariation of diversity with competition and crowding. We examined separately the effects of species richness and dominance on invasion by an exotic legume, Melilotus officinalis, in intact, native Kansas grassland. We manipulated dominance of C4 grasses by reducing their abundance (i.e. ramet densities) by ∼25 and 50%. In addition, richness was reduced by removing species that were mainly rare and uncommon as might be expected with environmental changes such as drought and fragmentation. In both years of the study (2001–2002), invasibility, measured as peak establishment of Melilotus, was not affected by a 3-fold reduction in species richness, nor was there an interaction between loss of species and reduced dominance on invasion. In contrast, reductions in abundance of the dominants significantly reduced invasibility of the grassland plots in both years. Because the abundance of dominants was highly correlated with measures of competition (i.e. ratio of dominant biomass to total biomass) and crowding (total stem densities), this pattern was opposite to that expected if competition were indeed limiting invasion. Rather, invasion appeared to be facilitated by the dominant species, most likely because reduced dominance increased environmental stress. Our results suggest that dominance is the key community characteristic determining invasibility, because highly competitive and space-filling species can either enhance or reduce susceptibility to invasion depending on whether dominants create a more competitive environment or alleviate stressful conditions. VL - 106 ER - TY - JOUR T1 - Generality in ecology: testing North American grassland rules in South African savannas JF - Frontiers in Ecology and the Environment Y1 - 2004 A1 - Alan K. Knapp A1 - M.D. Smith A1 - Scott. L. Collins A1 - Zambatis, N. A1 - Peel, M. A1 - Emery, S.M. A1 - Wojdak, J. A1 - Horner-Devine, M.C. A1 - Biggs, H. A1 - Kruger, J. A1 - Andelman, S.J. AB - Ecology has emerged as a global science, and there is a pressing need to identify ecological rules – general principles that will improve its predictive capability for scientists and its usefulness for managers and policy makers. Ideally, the generality and limits of these ecological rules should be assessed using extensive, coordinated experiments that ensure consistency in design and comparability of data. To improve the design of these large-scale efforts, existing data should be used to test prospective ecological rules and to identify their limits and contingencies. As an example of this approach, we describe prospective rules for grassland responses to fire and rainfall gradients, identified from long-term studies of North American grasslands and tested with existing data from long-term experiments in South African savanna grasslands. Analyses indicated consistent effects of fire on the abundance of the dominant (grasses) and subdominant (forbs) flora on both continents, but no common response of grass or forb abundance across a rainfall gradient. Such analyses can inform future research designs to refine and more explicitly test ecological rules. VL - 9 ER - TY - JOUR T1 - Invasion in space and time: non-native species richness and relative abundance respond to interannual variation in productivity and diversity JF - Ecology Letters Y1 - 2004 A1 - Cleland, E.E. A1 - M.D. Smith A1 - Andelman, S.J. A1 - Bowles, C. A1 - Carney, K.M. A1 - Horner-Devine, M.C. A1 - Drake, J.M. A1 - Emery, S.M. A1 - Gramling, J.M. A1 - Vandermast, D.B. AB - Ecologists have long sought to understand the relationships among species diversity, community productivity and invasion by non-native species. Here, four long-term observational datasets were analyzed using repeated measures statistics to determine how plant species richness and community resource capture (i.e. productivity) influenced invasion. Multiple factors influenced the results, including the metric used to quantify invasion, interannual variation and spatial scale. Native richness was positively correlated with non-native richness, but was usually negatively correlated with non-native abundance, and these patterns were stronger at the larger spatial scale. Logistic regressions indicated that the probability of invasion was reduced both within and following years with high productivity, except at the desert grassland site where high productivity was associated with increased invasion. Our analysis suggests that while non-natives were most likely to establish in species rich communities, their success was diminished by high resource capture by the resident community. VL - 7 ER - TY - JOUR T1 - Scale dependence in the relationship between species richness and productivity: the role of spatial and temporal turnover JF - Ecology Y1 - 2004 A1 - Chalcraft, D.R. A1 - Williams, J.W. A1 - M.D. Smith A1 - M.R. Whiles AB - Recent research in aquatic systems suggests that productivity–richness relationships change with spatial scale and that species turnover (i.e., spatial and temporal variation in species composition) plays an important role in generating this scale dependence. The generality of such scale dependence and the effects of variation in temporal scale remain unknown. We examined the extent to which the richness–productivity relationship in terrestrial plant communities depends on spatial or temporal scale and evaluated how spatial and temporal turnover (i.e., species turnover in space and time) generates scale dependence in these relationships using data from two Long-Term Ecological Research (LTER) sites (Jornada and Konza). We found a weak hump-shaped relationship (Jornada) and no relationship (Konza) between richness and productivity at the smallest focal scale (1 m2 at Jornada and 50 m2 at Konza) at each site, but strong hump-shaped relationships at the largest focal scale (49 m2 at Jornada and 200 m2 at Konza) for each site. Relationships between spatial turnover and productivity at each site mirrored the productivity–richness relationships that emerged at the larger spatial scale (i.e., a significant hump-shaped pattern). In contrast, temporal turnover was unrelated to productivity, and hence increasing temporal scale did not appreciably change the form of the productivity–richness relationship. Our study suggests that the way in which productivity–richness relationships change with spatial or temporal scale depends on the form and strength of the underlying relationship between species turnover and productivity. Moreover, we contend that a dominant effect of increasing productivity is the generation of dissimilarity in species composition among localities that comprise a region, rather than increasing the number of species that occur within local communities. Thus, understanding the mechanisms that cause species turnover to vary with productivity is critical to understanding scale dependence in richness–productivity relationships. VL - 85 ER - TY - JOUR T1 - Assessing the response of terrestrial ecosystems to potential changes in precipitation JF - BioScience Y1 - 2003 A1 - Weltzin, J.F. A1 - Loik, M.E. A1 - Schwinning, S. A1 - Williams, D.G. A1 - Fay, P.A. A1 - Haddad, B. A1 - Harte, J. A1 - Huxman, T.E. A1 - Alan K. Knapp A1 - Lin, G. A1 - Pockman, W.T. A1 - Shaw, M.R. A1 - Small, E. A1 - M.D. Smith A1 - Tissue, D.T. A1 - Zak, J.C. KW - Community KW - ecosystem KW - global change KW - Precipitation KW - soil moisture AB - Changes in Earth's surface temperatures caused by anthropogenic emissions of greenhouse gases are expected to affect global and regional precipitation regimes. Interactions between changing precipitation regimes and other aspects of global change are likely to affect natural and managed terrestrial ecosystems as well as human society. Although much recent research has focused on assessing the responses of terrestrial ecosystems to rising carbon dioxide or temperature, relatively little research has focused on understanding how ecosystems respond to changes in precipitation regimes. Here we review predicted changes in global and regional precipitation regimes, outline the consequences of precipitation change for natural ecosystems and human activities, and discuss approaches to improving understanding of ecosystem responses to changing precipitation. Further, we introduce the Precipitation and Ecosystem Change Research Network (PrecipNet), a new interdisciplinary research network assembled to encourage and foster communication and collaboration across research groups with common interests in the impacts of global change on precipitation regimes, ecosystem structure and function, and the human enterprise. VL - 53 ER - TY - JOUR T1 - Dominant species maintain ecosystem function with non-random species loss JF - Ecology Letters Y1 - 2003 A1 - M.D. Smith A1 - Alan K. Knapp AB - Loss of species caused by widespread stressors, such as drought and fragmentation, is likely to be non-random depending on species abundance in the community. We experimentally reduced the number of rare and uncommon plant species while independently reducing only the abundance of dominant grass species in intact, native grassland. This allowed us to simulate a non-random pattern of species loss, based on species abundances, from communities shaped by natural ecological interactions and characterized by uneven species abundance distributions. Over two growing seasons, total above-ground net primary productivity (ANPP) declined with reductions in abundance of the dominant species but was unaffected by a threefold decline in richness of less common species. In contrast, productivity of the remaining rare and uncommon species decreased with declining richness, in part due to loss of complementary interactions among these species. However, increased production of the dominant grasses offset the negative effects of species loss. We conclude that the dominant species, as controllers of ecosystem function, can provide short-term resistance to reductions in ecosystem function when species loss is nonrandom. However, the concurrent loss of complementary interactions among rare and uncommon species, the most diverse component of communities, may contribute to additional species loss and portends erosion of ecosystem function in the long term. VL - 6 ER - TY - THES T1 - Causes and consequences of species invasion and loss: the role of dominant species and diversity in maintaining ecosystem function Y1 - 2002 A1 - M.D. Smith PB - Kansas State University CY - Manhattan, KS VL - PhD Dissertation ER - TY - JOUR T1 - Rainfall variability, carbon cycling and plant species diversity in a mesic grassland JF - Science Y1 - 2002 A1 - Alan K. Knapp A1 - Fay, P.A. A1 - John M. Blair A1 - Scott. L. Collins A1 - M.D. Smith A1 - Carlisle, J.D. A1 - Harper, C.W. A1 - Danner, B.T. A1 - Lett, M.S. A1 - McCarron, J.K. AB -

Ecosystem responses to increased variability in rainfall, a prediction of general circulation models, were assessed in native grassland by reducing storm frequency and increasing rainfall quantity per storm during a 4-year experiment. More extreme rainfall patterns, without concurrent changes in total rainfall quantity, increased temporal variability in soil moisture and plant species diversity. However, carbon cycling processes such as soil CO2 flux, CO2 uptake by the dominant grasses, and aboveground net primary productivity (ANPP) were reduced, and ANPP was more responsive to soil moisture variability than to mean soil water content. Our results show that projected increases in rainfall variability can rapidly alter key carbon cycling processes and plant community composition, independent of changes in total precipitation.

VL - 298 UR - https://science.sciencemag.org/content/298/5601/2202 ER - TY - JOUR T1 - Effects of mycorrhizas on growth and demography of tallgrass prairie forbs JF - American Journal of Botany Y1 - 2001 A1 - G.T. Wilson A1 - D.C. Hartnett A1 - M.D. Smith A1 - Kobbeman, K. KW - demography KW - forb KW - tallgrass prairie AB - The effects of arbuscular mycorrhizal (AM) symbiosis on ramet and genet densities, vegetative growth rates, and flowering of three forb species were studied in native tallgrass prairie in northeastern Kansas. Mycorrhizal activity was experimentally suppressed for six growing seasons on replicate plots in an annually burned and an infrequently burned watershed with the fungicide benomyl. Benomyl reduced mycorrhizal root colonization to an average of 4.2%, approximately a two-thirds reduction relative to controls (13.7% colonization). Mycorrhizae influenced the population structure of these forbs. Although mycorrhizal suppression had no long-term effect on genet densities and no effect on ramet survivorship throughout the growing season, the number of ramets per individual was significantly increased such that ramet densities of all three species were approximately doubled in response to long-term mycorrhizal suppression. Effects of mycorrhizae on ramet growth and reproduction varied among species. Ramet growth rates, biomass, and flowering of Salvia azurea were greater in plots with active mycorrhizal symbiosis, whereas mycorrhizae reduced ramet growth rates and biomass of Artemesia ludoviciana. Aster sericeus ramet growth rates and biomass were unaffected by the fungicide applications, but its flowering was reduced.The pattern of responses of these three species to mycorrhizae differed considerably between the two sites of contrasting fire regime, indicating that the interaction of fire-induced shifts in resource availability and mycorrhizal symbiosis together modulates plant responses and the intensity and patterns of interspecific competition between and among tallgrass prairie grass and forb species. Further, the results indicate that effects of mycorrhizae on community structure are a result of interspecific differences in the balance between direct positive effects of the symbiosis on host plant performance and indirect negative effects mediated through altered competitive interactions. VL - 88 UR - http://www.ncbi.nlm.nih.gov/pubmed/21669678 ER - TY - Generic T1 - Interactions between fire and invasive plants in temperate grasslands of North America Y1 - 2001 A1 - Grace, J.B. A1 - M.D. Smith A1 - Grace, S.L. A1 - Scott. L. Collins A1 - Stohlgren, T.J. ED - Galley, K. ED - Wilson, T. KW - alien plants KW - exotic species KW - fire KW - Grasslands KW - invasive plants KW - prairie AB - A substantial number of invasive grasses, forbs and woody plants have invaded temperate grasslands in North America. Among the invading species are winter annuals, biennials, cool-season perennials, warm-season perennials, vines, shrubs, and trees. Many of these species have been deliberately introduced and widely planted; some are still used for range improvement, pastures, lawns, and as ornamentals, though many are listed as state or federal noxious weeds. Others have been greatly facilitated by widespread land disturbance. Historically, fire has been a major selective force in the evolution of temperate grasslands. Further, prescribed fire is commonly used as a method of ecological management for native grassland communities as well as in conjunction with restoration efforts. Within this context, it is important to understand how invasive species will interact with natural and prescribed fire regimes. In this paper, we consider what is known about how exotic species that invade temperate grasslands relate to fire. The primary issues addressed for each species are (1) Does fire appear to enhance colonization? (2) To what degree does fire affect the survival of plants? (3) Are plants that are burned able to regrow following fire and, if so, how rapidly can they recover? (4) How important is competition with native species to the response to fire? and (5) What effect does an invasive species have on the characteristics of the fire regime? For many species, results are preliminary, incomplete, or inconsistent among studies. For this reason, many of the conclusions drawn for individual species must be considered preliminary. Based on analyses of individual species, a conceptual framework is presented for considering how invasive plants may interact with fire when they invade an ecosystem. The major categories of influences are the native community, the fire regime, growth conditions for both invasive and native species, and influences that disturbances, human impacts, and landscape characteristics have had in the past and will have in the future. The examples considered in this paper provide support for a few, tentative generalizations. First, among our current worst invaders of temperate grasslands, adaptation to fire is quite variable. Some species are not well adapted to burning and can be easily eliminated; other species are better adapted but can still be eliminated if fire occurs during periods of particular vulnerability and/or at high frequency. There is a set of species that is extremely well adapted to fire and will not be eliminated through burning alone. Second, competitive interactions with native species play a crucial role in the success of nonnative invaders. In cases where differential burn responses between invasive and native species can be exploited, and adequate populations of native dominant species are present, fire can sometimes tip the competitive balance away from invasives. Third, there are a few invasive species that have exceptional attributes and for which there are no easy solutions. The ability of cheatgrass (Bromus tectorum) to enhance fire, the ability of Chinese tallow (Triadica sebifera) to suppress fire, and the ability of leafy spurge (Euphorbia esula) to resprout from repeated injury make the sespecies exceptional threats to native diversity. Finally, the available information for many invasive species is very incomplete, particularly with regard to how fire affects competitive interactions with the native community. There is much more we need to know if we are to consistently predict how invasive species will respond to fire and how burning can best be used to manage for natural diversity PB - Tall Timbers Research Station CY - Tallahassee, FL ER - TY - JOUR T1 - Physiological and morphological traits of exotic, invasive exotic, and native plant species in tallgrass prairie JF - International Journal of Plant Sciences Y1 - 2001 A1 - M.D. Smith A1 - Alan K. Knapp KW - biomass allocation KW - grassland KW - invasive plants KW - photosynthesis KW - specific leaf area AB - We compared 13 traits of invasive exotic, noninvasive exotic, and ecologically similar native species to determine if there are generalizable differences among these groups that relate to persistence and spread of exotic species in tallgrass prairie plant communities. When species were grouped as invasive (two species), noninvasive (five species), and native (six species), no differences were found for the suite of traits examined, likely because of the high variability within and between groups. However, when exotic species, regardless of invasiveness, were compared with the native species, specific leaf area was ca. 40% higher for the exotic species, a result that is consistent with that of other studies. This pattern was also observed for five of seven pairwise comparisons of exotic and native species with similar life history traits. In contrast, total end‐of‐season biomass was as much as three times higher for the native species in five of seven of the native‐exotic species pairs. For other traits, differences between exotic and native species were species‐specific and were generally more numerous for noninvasive than for invasive exotic species pair‐wise comparisons. Thus, contrary to predictions, exotic species capable of successfully invading tallgrass prairie did not differ considerably from native species in most traits related to resource utilization and carbon gain. Moreover, invasive exotic species, those capable of displacing native species and dominating a community, were not distinct for the observed traits from their native counterparts. These results indicate that other traits, such as the ability to respond to resource pulses or herbivory, may explain more effectively why certain invasive species are able to invade these communities aggressively. VL - 162 ER - TY - JOUR T1 - Size of the local species pool determines invasibility of a C4-dominated grassland JF - Oikos Y1 - 2001 A1 - M.D. Smith A1 - Alan K. Knapp AB - The size of the local species pool (i.e., species surrounding a community capable of dispersal into that community) and other dispersal limitations strongly influence native plant community composition. However, the role that the local species pool plays in determining the invasibility of communities by exotic plants remains to be evaluated. We hypothesized that the richness and abundance of exotic species would be greater in C4-dominated grassland communities if the local species pool included a larger proportion of exotic species. We also predicted that an increase in the exotic species pool would increase the invasibility of sites thought to be resistant to invasion (annually burned grassland). To test these hypotheses, study plots were established within two long-term (>20 yr) fire experiments at a tallgrass prairie preserve in NE Kansas (USA). Study plots were surrounded by either a small pool of exotic species (small species pool (SSP) plots; six species) or a larger exotic species pool (large species pool (LSP) plots; 18 species). We found that richness and absolute cover of exotic species was significantly (P<0.001) lower (∼70 and 90%, respectively) in annually burned compared to unburned plots, regardless of the size of the exotic species pool. As predicted, exotic species richness was higher (P<0.001) for LSP plots (3.9 per 250 m2) than for SSP plots (0.7 per 250 m2); however, absolute cover was unaffected by the size of the exotic species pool. In the absence of fire, plots with a LSP had four times as many exotic species than SSP plots. An increase in the local exotic species pool also increased the invasibility of annually burned grassland. Indeed, richness of exotic plant species in annually burned LSP plots did not differ from unburned plots with a SSP, indicating that a larger pool of exotic species countered the negative effects of fire. These findings have important implications for predicting how the invasion of plant communities may respond to human-induced global changes, such as habitat fragmentation. Community characteristics or factors such as frequent fires in grasslands may impart resistance to invasions by exotic species in large, intact ecosystems. However, when a large pool of exotic species is present, frequent fire may not be sufficient to limit the invasions of exotic plants in fragmented landscapes. VL - 92 ER - TY - JOUR T1 - Variation among biomes in temporal dynamics of aboveground primary production JF - Science Y1 - 2001 A1 - Alan K. Knapp A1 - M.D. Smith KW - Primary production AB - Interannual variability in aboveground net primary production (ANPP) was assessed with long-term (mean = 12 years) data from 11 Long Term Ecological Research sites across North America. The greatest interannual variability in ANPP occurred in grasslands and old fields, with forests the least variable. At a continental scale, ANPP was strongly correlated with annual precipitation. However, interannual variability in ANPP was not related to variability in precipitation. Instead, maximum variability in ANPP occurred in biomes where high potential growth rates of herbaceous vegetation were combined with moderate variability in precipitation. In the most dynamic biomes, ANPP responded more strongly to wet than to dry years. Recognition of the fourfold range in ANPP dynamics across biomes and of the factors that constrain this variability is critical for detecting the biotic impacts of global change phenomena. VL - 291 ER - TY - JOUR T1 - Effects of long-term fungicide application on microbial processes in tallgrass prairie soils JF - Soil Biology & Biochemistry Y1 - 2000 A1 - M.D. Smith A1 - D.C. Hartnett A1 - C. W. Rice KW - Arbuscular mycorrhizal fungi KW - Benomyl KW - carbon KW - Microbial biomass KW - Nematodes KW - nitrogen AB - Several studies investigating the role of arbuscular mycorrhizal (AM) fungi in plant communities have included manipulations of AM fungal symbiosis using the fungicide benomyl. The objectives of this study were to evaluate the potential non-target effects of benomyl on soil biota and nutrient cycling in tallgrass prairie and to determine how mycorrhizae may influence these belowground properties. To accomplish these objectives, soil samples were collected during the 1996–1997 growing seasons from long-term benomyl-treated plots established on tallgrass prairie (Manhattan, KS) in 1991, and the following measurements were made: total bacterial and fungal biomass; abundance of nematodes; microbial biomass carbon and nitrogen; substrate-induced respiration; and potentially mineralizable C and N. Long-term benomyl applications resulted in an 80% reduction in mycorrhizal root colonization. By reducing root colonization, benomyl applications resulted in significant decreases in total bacterial biomass and abundance of fungal-feeding and predatory nematodes (20, 12 and 33% reductions compared to control, respectively). Total microbial potential activity (i.e., substrate-induced respiration) increased by 10% with benomyl treatment, whereas the relative contribution of fungi to total microbial activity decreased significantly with benomyl applications. In addition, microbial biomass C increased from 1364 (± 51.2 SE) to 1485 (± 51.2 SE) with benomyl treatment, and total carbon increased significantly (∼8%) only in annually burned soils treated with benomyl. The magnitude of benomyl effects on soil components and processes were small (<33% change with benomyl) relative to effects on mycorrhizal root colonization (80% decrease with benomyl). These results indicate that rather than having large non-target effects, benomyl applications principally affect mycorrhizal root colonization, thereby indirectly influencing soil biota and nutrient availability. Results also indicate that mycorrhizal fungi play an important role in altering the availability and flow of carbon in prairie soil and may influence the composition and abundance of groups of some soil biota. Changes in soil organisms and nutrient availability associated with altered mycorrhizal symbiosis may influence aboveground plant species responses to mycorrhizal suppression, but further research is needed to understand these potential effects. VL - 32 ER - TY - JOUR T1 - Exotic plant species in a C4-dominated grassland: Invasibility, disturbance and community structure JF - Oecologia Y1 - 1999 A1 - M.D. Smith A1 - Alan K. Knapp KW - community structure KW - disturbance KW - exotic species KW - grassland KW - Invasibility AB - We used data from a 15-year experiment in a C4-dominated grassland to address the effects of community structure (i.e., plant species richness, dominance) and disturbance on invasibility, as measured by abundance and richness of exotic species. Our specific objectives were to assess the temporal and spatial patterns of exotic plant species in a native grassland in Kansas (USA) and to determine the factors that control exotic species abundance and richness (i.e., invasibility). Exotic species (90% C3 plants) comprised approximately 10% of the flora, and their turnover was relatively high (30%) over the 15-year period. We found that disturbances significantly affected the abundance and richness of exotic species. In particular, long-term annually burned watersheds had lower cover of exotic species than unburned watersheds, and fire reduced exotic species richness by 80–90%. Exotic and native species richness were positively correlated across sites subjected to different fire (r = 0.72) and grazing (r = 0.67) treatments, and the number of exotic species was lowest on sites with the highest productivity of C4 grasses (i.e., high dominance). These results provide strong evidence for the role of community structure, as affected by disturbance, in determining invasibility of this grassland. Moreover, a significant positive relationship between exotic and native species richness was observed within a disturbance regime (annually burned sites, r = 0.51; unburned sites, r = 0.59). Thus, invasibility of this C4-dominated grassland can also be directly related to community structure independent of disturbance. VL - 120 ER - TY - JOUR T1 - Interacting influence of mycorrhizal symbiosis and competition on plant diversity in tallgrass prairie JF - Oecologia Y1 - 1999 A1 - M.D. Smith A1 - D.C. Hartnett A1 - G.T. Wilson KW - Arbuscular mycorrhizal fungi KW - Competition KW - diversity KW - Species coexistence KW - tallgrass prairie AB - In tallgrass prairie, plant species interactions regulated by their associated mycorrhizal fungi may be important forces that influence species coexistence and community structure; however, the mechanisms and magnitude of these interactions remain unknown. The objective of this study was to determine how interspecific competition, mycorrhizal symbiosis, and their interactions influence plant community structure. We conducted a factorial experiment, which incorporated manipulations of abundance of dominant competitors, Andropogon gerardii and Sorghastrum nutans, and suppression of mycorrhizal symbiosis using the fungicide benomyl under two fire regimes (annual and 4-year burn intervals). Removal of the two dominant C4 grass species altered the community structure, increased plant species richness, diversity, and evenness, and increased abundance of subdominant graminoid and forb species. Suppression of mycorrhizal fungi resulted in smaller shifts in community structure, although plant species richness and diversity increased. Responses of individual plant species were associated with their degree of mycorrhizal responsiveness: highly mycorrhizal responsive species decreased in abundance and less mycorrhizal responsive species increased in abundance. The combination of dominant-grass removal and mycorrhizal suppression treatments interacted to increase synergistically the abundance of several species, indicating that both processes influence species interactions and community organization in tallgrass prairie. These results provide evidence that mycorrhizal fungi affect plant communities indirectly by influencing the pattern and strength of plant competitive interactions. Burning strongly influenced the outcome of these interactions, which suggests that plant species diversity in tallgrass prairie is influenced by a complex array of interacting processes, including both competition and mycorrhizal symbiosis. VL - 121 ER - TY - THES T1 - The role of mycorrhizae and dominant competitors in tallgrass prairie plant community structure and belowground processes Y1 - 1998 A1 - M.D. Smith KW - tallgrass prairie PB - Kansas State University CY - Manhattan, KS VL - MS Thesis ER -