@article {6068, title = {Repeated extreme droughts decrease root production, but not the potential for post-drought recovery of root production, in a mesic grassland}, journal = {Oikos}, volume = {1}, year = {2023}, pages = {e08899}, abstract = {

Global climate change is expected to cause more frequent extreme droughts in many parts of the world. Despite the crucial role of roots in water acquisition and plant survival, our understanding of ecosystem vulnerability to drought is primarily based on aboveground impacts. As return intervals between droughts decrease, root responses to one drought might alter responses to subsequent droughts, but this remains unresolved. We conducted a seven-year experiment that imposed extreme drought (growing season precipitation reduced 66\%) in a mesic grassland. Plots were droughted during years 1\–2 (\‘Drought 1\&$\#$39;), or years 5\–6 (\‘Drought 2\&$\#$39;) or both. We quantified root production during year 6 (final year of Drought 2) and year 7 (first year after Drought 2), when all plots received ambient precipitation. We found that repeated drought decreased root mass production more than twice as much as a single drought (\−63\% versus \−27\%, respectively, relative to ambient precipitation). Root mass production of the dominant C4 grass Andropogon gerardii did not decrease significantly with either one or two droughts. A. gerardii root traits differed from subdominant species on average across all treatments, but drought did not alter root traits of either A. gerardii or the subdominant species (collectively). In year 6, root production in plots droughted 4 years ago had not recovered (\−21\% versus control), but root production recovered in all formerly droughted plots in year 7, when precipitation was above average. Our results highlight the complexity of root responses to drought. Drought-induced reductions in root production can persist for years after drought and repeated drought can reduce production even further, but this does not preclude rapid recovery of root production in a wet year.

}, keywords = {LTER-KNZ}, doi = {10.1111/oik.08899}, url = {https://onlinelibrary.wiley.com/doi/10.1111/oik.08899}, author = {Slette, I.J. and D.L. Hoover and M. D. Smith and Knapp, A.K.} } @article {KNZ002014, title = {Rainfall-manipulation experiments as simulated by terrestrial biosphere models: where do we stand?}, journal = {Global Change Biology}, volume = {26}, year = {2020}, pages = {3336{\textendash}3355}, abstract = {

Changes in rainfall amounts and patterns have been observed and are expected to continue in the near future with potentially significant ecological and societal consequences. Modelling vegetation responses to changes in rainfall is thus crucial to project water and carbon cycles in the future. In this study, we present the results of a new model-data intercomparison project, where we tested the ability of ten terrestrial biosphere models to reproduce observed sensitivity of ecosystem productivity to rainfall changes at ten sites across the globe, in nine of which, rainfall exclusion and/or irrigation experiments had been performed.

The key results are:
(a) Inter-model variation is generally large and model agreement varies with time scales. In severely water limited sites, models only agree on the interannual variability of evapotranspiration and to a smaller extent gross primary productivity. In more mesic sites model agreement for both water and carbon fluxes is typically higher on fine (daily-monthly) time scales and reduces on longer (seasonal-annual) scales.
(b) Models on average overestimate the relationship between ecosystem productivity and mean rainfall amounts across sites (in space) and have a low capacity in reproducing the temporal (interannual) sensitivity of vegetation productivity to annual rainfall at a given site, even though observation uncertainty is comparable to inter-model variability.
(c) Most models reproduced the sign of the observed patterns in productivity changes in rainfall manipulation experiments but had a low capacity in reproducing the observed magnitude of productivity changes. Models better reproduced the observed productivity responses due to rainfall exclusion than addition.
(d) All models attribute ecosystem productivity changes to the intensity of vegetation stress and peak leaf area, whereas the impact of the change in growing season length is negligible. The relative contribution of the peak leaf area and vegetation stress intensity was highly variable among models.

}, keywords = {LTER-KNZ}, doi = {10.1111/gcb.15024}, url = {https://onlinelibrary.wiley.com/doi/abs/10.1111/gcb.15024}, author = {Paschalis, Athanasios and Fatichi, Simone and Zscheischler, Jakob and Ciais, Philippe and Michael Bahn and Lena R. Boysen and Chang, Jinfeng and De Kauwe, Martin and Estiarte, Marc and Goll, Daniel and Hanson, Paul J. and Harper, Anna B. and Hou, Enqing and Kigel, Jaime and Alan K. Knapp and Larsen, Klaus Steenberg and Li, Wei and Lienert, Sebastian and Luo, Yiqi and Meir, Patrick and Nabel, Julia E.M.S. and Ogaya, Rom{\`a} and Parolari, Anthony J and Peng, Changhui and Pe{\~n}uelas, Josep and Pongratz, Julia and Rambal, Serge and Schmidt, Inger Kappel and Shi, Hao and Sternberg, Marcelo and Tian, Hanqin and Tschumi, Elisabeth and Ukkola, Anna and Vicca, Sara and Viovy, Nicolas and Wang, -Ping and Wang, Zhuonan and Williams, Karina and Wu, Donghai and Zhu, Qiuan} } @article {KNZ001887, title = {A reality check for climate change experiments: Do they reflect the real world?}, journal = {Ecology}, volume = {99}, year = {2018}, pages = {2145-2151}, abstract = {

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.

}, keywords = {LTER-KNZ}, doi = {10.1002/ecy.2474}, url = {https://esajournals.onlinelibrary.wiley.com/doi/epdf/10.1002/ecy.2474}, author = {Alan K. Knapp and Carroll, C.J.W. and Griffin-Nolan, R.J. and Slette, I.J. and Chavez, F.A. and Baur, L. and Felton, A.J. and Gray, J. and Hoffman, A.M. and Lemoine, N.P. and Mao, W. and Post, A. and M.D. Smith} } @article {KNZ001801, title = {Reconciling inconsistencies in precipitation{\textendash} productivity relationships: implications for climate change}, journal = {New Phytologist}, volume = {214}, year = {2017}, pages = {41-47}, abstract = {Precipitation (PPT) is a primary climatic determinant of plant growth and aboveground net primary production (ANPP) over much of the globe. Thus, PPT{\textendash}ANPP relationships are important both ecologically and to land{\textendash}atmosphere models that couple terrestrial vegetation to the global carbon cycle. Empirical PPT{\textendash}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{\textendash}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.}, keywords = {LTER-KNZ}, doi = {10.1111/nph.14381}, url = {https://nph.onlinelibrary.wiley.com/doi/full/10.1111/nph.14381}, author = {Alan K. Knapp and Ciais, P. and M.D. Smith} } @article {KNZ001781, title = {Rangeland responses to predicted increases in drought extremity}, journal = {Rangelands }, volume = {38}, year = {2016}, pages = {191-196}, keywords = {LTER-KNZ, Drought, grassland, savanna, shrubland, state and transition, woodland}, url = {http://dx.doi.org/10.1016/j.rala.2016.06.009}, author = {Breshears, D.D. and Alan K. Knapp and Law, D.J. and M.D. Smith and Twidwell, D. and Wonkka, C.L.} } @article {KNZ001532, title = {Rainfall variability has minimal effects on grassland recovery from repeated grazing}, journal = {Journal of Vegetation Science}, volume = {25}, year = {2014}, pages = {36 -44}, abstract = {

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.

}, keywords = {LTER-KNZ, Annual net primary production, Climate change, Clipping, disturbance, diversity, Grass{\textendash}forb interaction, Konza, Precipitation, Stem density}, doi = {10.1111/jvs.12065}, url = {https://onlinelibrary.wiley.com/doi/abs/10.1111/jvs.12065}, author = {Koerner, S.E. and Scott. L. Collins and John M. Blair and Alan K. Knapp and M.D. Smith} } @article {KNZ001613, title = {Resistance and resilience of a grassland ecosystem to climate extremes}, journal = {Ecology}, volume = {95}, year = {2014}, pages = {2646 -2656}, abstract = {

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.

}, keywords = {LTER-KNZ}, doi = {10.1890/13-2186.1}, url = {https://esajournals.onlinelibrary.wiley.com/doi/abs/10.1890/13-2186.1}, author = {D.L. Hoover and Alan K. Knapp and M.D. Smith} } @article {KNZ001587, title = {Responses to fire differ between South African and North American grassland communities}, journal = {Journal of Vegetation Science}, volume = {25}, year = {2014}, pages = {793 -804}, abstract = {

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.

}, keywords = {LTER-KNZ, Community ecology, Divergence, Fire frequency, Konza Prairie Biological Station, Mesic grassland, nitrogen, Nutrient addition, Richness, tallgrass prairie, Ukulinga Research Farm}, doi = {10.1111/jvs.12130}, url = {https://onlinelibrary.wiley.com/doi/abs/10.1111/jvs.12130}, author = {Kirkman, K. and Scott. L. Collins and M.D. Smith and Alan K. Knapp and Burkepile, D.E. and Burns, C.E. and Fynn, R.W.S. and Hagenah, N. and Koerner, S.E. and Matchett, K.J. and Thompson, D.I. and K.R. Wilcox and Wragg, P.D.} } @article {KNZ001449, title = {Relative effects of precipitation variability and warming on tallgrass prairie ecosystem function}, journal = {Biogeosciences}, volume = {8}, year = {2011}, pages = {3053 -3068}, abstract = {

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.

}, keywords = {LTER-KNZ}, doi = {10.5194/bg-8-3053-2011}, url = {https://www.biogeosciences.net/8/3053/2011/}, author = {Fay, P.A. and John M. Blair and M.D. Smith and Jesse B. Nippert and Carlisle, J.D. and Alan K. Knapp} } @article {KNZ00893, title = {Regional patterns in carbon cycling across the Great Plains of North America}, journal = {Ecosystems}, volume = {8}, year = {2005}, pages = {106 -121}, abstract = {The large organic carbon (C) pools found in noncultivated grassland soils suggest that historically these ecosystems have had high rates of C sequestration. Changes in the soil C pool over time are a function of alterations in C input and output rates. Across the Great Plains and at individual sites through time, inputs of C (via aboveground production) are correlated with precipitation; however, regional trends in C outputs and the sensitivity of these C fluxes to annual variability in precipitation are less well known. To address the role of precipitation in controlling grassland C fluxes, and thereby soil C sequestration rates, we measured aboveground and belowground net primary production (ANPP-C and BNPP-C), soil respiration (SR-C), and litter decomposition rates for 2 years, a relatively dry year followed by a year of average precipitation, at five sites spanning a precipitation gradient in the Great Plains. ANPP-C, SR-C, and litter decomposition increased from shortgrass steppe (36, 454, and 24 g C m-2 y-1) to tallgrass prairie (180, 1221, and 208 g C m-2 y-1 for ANPP-C, SR-C, and litter decomposition, respectively). No significant regional trend in BNPP-C was found. Increasing precipitation between years increased rates of ANPP-C, BNPP-C, SR-C, and litter decomposition at most sites. However, regional patterns of the sensitivity of ANPP-C, BNPP-C, SR-C, and litter decomposition to between-year differences in precipitation varied. BNPP-C was more sensitive to between-year differences in precipitation than were the other C fluxes, and shortgrass steppe was more responsive than were mixed grass and tallgrass prairie.}, keywords = {LTER-KNZ, grassland, Great Plains, litter decomposition, Net primary production, Precipitation, Regional trends, soil respiration}, doi = {10.1007/s10021-004-0117-8}, author = {McCulley, R.L. and Burke, I.C. and Nelson, J.A. and Lauenroth, W.K. and Alan K. Knapp and Kelly, E.F.} } @inbook {KNZ00847, title = {Rainfall timing, soil moisture dynamics, and plant responses in a mesic tallgrass prairie ecosystem}, booktitle = {Precipitation Regimes and Terrestrial Ecosystems. A North American Perspective}, year = {2003}, pages = {147 -163}, publisher = {University of Arizona Press}, organization = {University of Arizona Press}, address = {Tucson, AZ}, keywords = {LTER-KNZ, tallgrass prairie}, author = {P. Fay and Alan K. Knapp and John M. Blair and Carlisle, J.D. and Danner, B.T. and McCarron, J.K.}, editor = {Weltzin, J.F. and McPherson, G.R.} } @article {KNZ00827, title = {Rainfall variability, carbon cycling and plant species diversity in a mesic grassland}, journal = {Science}, volume = {298}, year = {2002}, pages = {2202 -2205}, abstract = {

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.

}, keywords = {LTER-KNZ}, doi = {10.1126/science.1076347}, url = {https://science.sciencemag.org/content/298/5601/2202}, author = {Alan K. Knapp and Fay, P.A. and John M. Blair and Scott. L. Collins and M.D. Smith and Carlisle, J.D. and Harper, C.W. and Danner, B.T. and Lett, M.S. and McCarron, J.K.} } @article {KNZ00787, title = {Responses of the co-dominant grassland species Andropogon gerardii and Sorghastrum nutans to long-term manipulations of nitrogen and water}, journal = {American Midland Naturalist}, volume = {145}, year = {2001}, pages = {159 -167}, abstract = {The availability and distribution of essential resources such as water and nitrogen are changing on a global scale and it is important to determine the effects of such alterations on dominant and subdominant species in ecosystems. We examined the effects of long-term additions of water and nitrogen on leaf-level ecophysiological characteristics of the tallgrass prairie codominants Andropogon gerardii (big bluestem) and Sorghastrum nutans (Indian grass). Both species responded to irrigation with increased water potentials and to fertilization with decreased water potentials. Only in S. nutans, however, were net photosynthesis and stomatal conductance greater in response to both water and nitrogen addition. In contrast, A. gerardii, which currently is more abundant than S. nutans, was relatively unresponsive to resource manipulations. If net photosynthesis is positively related to plant productivity, as has been shown for tallgrass prairie species, the differential photosynthetic responses of these two grasses suggests that their patterns of abundance may change in the future if resource availability is altered. These results are also inconsistent with the historical assumption that these two grasses are {\textquotedblleft}ecological equivalents.{\textquotedblright}}, keywords = {LTER-KNZ}, doi = {10.1674/0003-0031(2001)145[0159:ROTCGS]2.0.CO;2}, author = {Silletti, A.M. and Alan K. Knapp} } @inbook {KNZ00646, title = {Regional climate and the distribution of tallgrass prairie}, booktitle = {Grassland Dynamics: Long-Term Ecological Research in Tallgrass Prairie}, year = {1998}, pages = {19 -34}, publisher = {Oxford University Press}, organization = {Oxford University Press}, address = {New York}, keywords = {LTER-KNZ, tallgrass prairie}, url = {http://www.colostate.edu/Depts/GDPE/Distinguished_Ecologists/2005/Hayden/grassland\%20dynamics\%20ch2.pdf}, author = {Hayden, B.P.}, editor = {Alan K. Knapp and J. M. Briggs and D.C. Hartnett and Scott. L. Collins} } @article {KNZ00621, title = {Responses of soil respiration to clipping and grazing in a tallgrass prairie}, journal = {Journal of Environmental Quality}, volume = {27}, year = {1998}, pages = {1539 -1548}, abstract = {Soil-surface CO2 flux (Fs) is an important component in prairie C budgets. Although grazing is common in grasslands, its effects on Fs have not been well documented. Three clipping treatments: (i) early-season clipping (EC); (ii) full-season clipping (FC); and (iii) no clipping (NC); which represented two grazing strategies and a control, were applied to plots in a tallgrass prairie in northeastern Kansas, USA. Measurements of Fs were made with a portable gas-exchange system at weekly to monthly intervals for 1 yr. Concurrent measurements of soil temperature and volumetric soil water content at 0.1 m were obtained with dual-probe heat-capacity sensors. Measurements of Fs also were obtained in grazed pastures. Fs ranged annually from 8.8 {\texttimes} 10-3 mg m-2 S-1 during the winter to 0.51 mg m-2 s-1 during the summer, following the patterns of soil temperature and canopy growth and phenology. Clipping typically reduced Fs 21 to 49\% by the second day after clipping despite higher soil temperatures in clipped plots. Cumulative annual Fs were 4.94, 4.04, and 4.11 kg m-2 yr-1 in NC, EC, and FC treatments, respectively; thus, dipping reduced annual Fs by 17.5\%. Differences in Fs between EC and FC were minimal, suggesting that different grazing strategies had little additional impact on annual Fs. Daily Fs in grazed pastures was 20 to 37\% less than Fs in ungrazed pastures. Results suggest that grazing moderates Fs during the growing season by reducing canopy photosynthesis and slowing translocation of carbon to the rhizosphere.}, keywords = {LTER-KNZ, tallgrass prairie}, doi = {10.2134/jeq1998.00472425002700060034x}, author = {D. Bremer and J.M. Ham and Owensby, C.E. and Alan K. Knapp} } @article {KNZ00638, title = {Responses to short-term shade in soybean leaves: Effects of leaf position and drought stress}, journal = {International Journal of Plant Science}, volume = {159}, year = {1998}, pages = {805 -811}, abstract = {This study examined how leaf position in the canopy affected photosynthetic and stomatal responses to short-term, minutes-long shade periods during a drought cycle in soybean (Glycine max [L.] Merr.). All soybean leaves had similar basic responses to short-term shade, including rapidly decreased photosynthetic rates (Aco2 ), slower decreases in stomatal conductance (g,), and delayed stomatal reopening and photosynthetic recovery after leaves were reilluminated. Drought stress lowered overall Aco2 and restricted photosynthetic and stomatal responses to short-term shade with the negative effects of drought being stronger in lower than in upper leaves. Some of the negative effects of drought persisted after drought was relieved, causing reduced overall water use efficiency, especially in lower leaves. These results indicate that leaf position effects on stomatal responses to short-term shade events become important during and after stress.}, keywords = {LTER-KNZ}, url = {http://www.jstor.org/stable/2475151}, author = {Fay, P.A. and Alan K. Knapp} } @article {KNZ00556, title = {Responses in stomatal conductance to elevated CO2 in 12 grassland species that differ in growth form}, journal = {Vegetatio}, volume = {125}, year = {1996}, pages = {31 -41}, abstract = {Responses in stomatal conductance (g st ) and leaf xylem pressure potential (ψ leaf ) to elevated CO2 (2x ambient) were compared among 12 tallgrass prairie species that differed in growth form and growth rate. Open-top chambers (OTCs, 4.5 m diameter, 4.0 m in height) were used to expose plants to ambient and elevated CO2 concentrations from April through November in undisturbed tallgrass prairie in NE Kansas (USA). In June and August, ψ leaf was usually higher in all species at elevated CO2 and was lowest in adjacent field plots (without OTCs). During June, when water availability was high, elevated CO2 resulted in decreased g st in 10 of the 12 species measured. Greatest decreases in g st (ca. 50\%) occurred in growth forms with the highest potential growth rates (C3 and C4 grasses, and C3 ruderals). In contrast, no significant decrease in g st was measured in the two C3 shrubs. During a dry period in September, reductions in g st at elevated CO2 were measured in only two species (a C3 ruderal and a C4 grass) whereas increased g st at elevated CO2 was measured in the shrubs and a C3 forb. These increases in g st were attributed to enhanced ψ leaf in the elevated CO2 plants resulting from increased soil water availability and/or greater root biomass. During a wet period in September, only reductions in g st were measured in response to elevated CO2. Thus, there was significant interspecific variability in stomatal responses to CO2 that may be related to growth form or growth rate and plant water relations. The effect of growth in the OTCs, relative to field plants, was usually positive for g st and was greatest (>30\%) when water availability was low, but only 6{\textendash}12\% when ψ leaf was high. The results of this study confirm the importance of considering interactions between indirect effects of high CO2 of plant water relations and direct effects of elevated CO2 on g st , particularly in ecosystems such as grasslands where water availability often limits productivity. A product of this interaction is that the potential exists for either positive or negative responses in g st to be measured at elevated levels of CO2.}, keywords = {LTER-KNZ, elevated CO2, Growth form, Stomatal Conductance, tallgrass prairie, Water relations}, doi = {10.1007/BF00045202}, author = {Alan K. Knapp and Hamerlynck, E.P. and J.M. Ham and Owensby, C.E.} } @article {KNZ00574, title = {Responses of a C4 grass and three C3 forbs to variation in nitrogen and light in tallgrass prairie}, journal = {Ecology}, volume = {77}, year = {1996}, pages = {1738 -1749}, abstract = {In tallgrass prairie, high plant species diversity results not from a large number of grass species, but from a large number of forb (nongrass, herbaceous) species. Forbs exhibit morphological, life history, and ecophysiological characteristics that contrast sharply with those of the dominant C4 grasses. Success of the subdominant forbs varies strongly with topographic position and burning regime, and landscape scale patterns of abundance are well documented. But comparatively little is known about the mechanisms determining these patterns in persistent tallgrass prairie forbs. To elucidate these mechanisms, (1) leaf{\textemdash}level physiological characteristics of the dominant C4 grass, Andropogon gerardii, and four co{\textemdash}occurring C3 forbs were measured in response to natural and experimentally manipulated gradients of N availability, and (2) seasonal light environments of forbs in contrasting topographic positions and burning regimes and their morphological and physiological responses in these environments were compared to determine whether resource availability and utilization patterns contributed to patterns of forb distribution and abundance. The effects of burning regime and topographic position on maximum rate of photosynthesis (A) and stomatal conductance to water vapor (g) measured at the leaf level were not consistent with patterns of forb abundance. Nitrogen did not appear to limit forb physiological processes, even though increased N availability resulted in higher tissue N concentrations and greater biomass. There was no consistent increase in (A) or decrease in (g) in response to fertilization. However, (A) at low light levels was as much as 67\% higher in fertilized Vernonia baldwinii and A. gerardii compared to unfertilized plants. Greater light availability to forbs in the canopy was associated with lower grass biomass production in uplands compared to lowlands and in unburned compared to burned sites. Forbs did not appear to adjust morphologically (leaf area and plant height) to different light environments at different sites. As a result, as much as 90\% of forb leaf area in the burned lowland was displayed in low light, whereas as little as 30\% of forb leaf area was in low light in the uplands at midseason. Estimates of potential whole{\textemdash}plant carbon uptake, based on leaf area distribution relative to available light and (A) as a function of light availability, agreed well with patterns of forb abundance and production. Differences in light availability may account for much of the variability in forb abundance related to burning regime and topographic position by limiting carbon gain in forbs more in burned lowlands than in other sites.}, keywords = {LTER-KNZ, tallgrass prairie}, doi = {10.2307/2265779}, author = {Turner, C.T. and Alan K. Knapp} } @proceedings {KNZ00502, title = {The role of fire in Spartina pectinata wetlands dominated tallgrass prairie wetlands}, year = {1995}, pages = {92 -101}, publisher = {Tall Timbers Research Station}, address = {Tallahassee, FL}, keywords = {LTER-KNZ, tallgrass prairie}, author = {Johnson, S.R. and Alan K. Knapp}, editor = {Cerulean, S.I. and Engstroms, R.T.} } @article {KNZ00439, title = {Responses of forbs and grasses to selective grazing by bison: interactions between herbivory and water stress}, journal = {Vegetatio}, volume = {115}, year = {1994}, pages = {123 -131}, abstract = {Two abundant tallgrass prairie forb species, Ambrosia psilostachya and Vernonia baldwinii, are commonly found intact in patches where the grasses have been selectively grazed by bison. Microclimatic patterns and physiological responses of these forbs were measured in grazed and ungrazed patches. These experiments demonstrated that bison herbivory indirectly enhanced water availability and productivity of forbs growing in grazed patches. This was due primarily to the reduction in transpiring grass leaf area in grazed patches and an increase in light availability. In grazed patches, incident light at forb mid-canopy height was 53\% greater than ungrazed sites at midseason and soil temperatures were always warmer (e.g., 10 ?C at 5 cm), perhaps enabling forbs to initiate growth earlier in the spring. Enhanced leaf xylem pressure potential and stomatal conductance in plants in grazed areas were most evident when water availability was low (i.e., late in the growing season and over short-term dry periods characteristic of the tallgrass prairie environment). Relative to individuals in ungrazed areas, end-of-season biomass of A. psilostachya was 40\% greater and reproductive biomass and head number of V. baldwinii was 45\% and 40\% greater, respectively, in plants in grazed patches. A favorable growing environment maintained in grazed patches during periods of water limitation enhances carbon gain in forbs leading to increased biomass and potential fitness.}, keywords = {LTER-KNZ, bison, Light reduction, Selective herbivory, tallgrass prairie, water stress}, doi = {10.1007/BF00044867}, url = {http://link.springer.com/article/10.1007/BF00044867}, author = {Fahnestock, J.T. and Alan K. Knapp} } @article {KNZ00274, title = {Response of Andropogon gerardii to simulated acid rain}, journal = {Transactions of the Kansas Academy of Science}, volume = {93}, year = {1990}, pages = {85 -90}, abstract = {

In this study the authors examined the effects that moderate exposure to acidic precipation had on growth and physiological responses in Andropogon gerardii, the dominant grass in tallgrass prairie. Naturally occurring monocultures of A. gerardii were treated with simulated acid rain (pH 4.0 or 3.0) or precipitation with a pH similar to natural rainfall (pH 5.0) throughout a growth season. No visual effects of the acid rain treatments were observed and there were no statistically significant effects on net photosynthetic CO2 uptake, stomatal conductance to water vapor, or tiller mass in A. gerardii. However, tiller density was significantly reduced as treatment pH decreased. These data suggest that a moderate increase in the number of rainfall events with low pH will have little discernible short-term impact on tallgrass prairie dominated by A. gerardii, but longer-term effects need to be evaluated

}, keywords = {LTER-KNZ, tallgrass prairie}, doi = {10.2307/3628150}, author = {Fahnestock, J.T. and Alan K. Knapp} } @article {KNZ0090, title = {Response of Andropogon gerardii (poaceae ) to fire-induced high vs.low irradiance environments in tallgrass prairie: leaf structure and photosynthetic pigments}, journal = {American Journal of Botany}, volume = {72}, year = {1985}, pages = {1668 -1671}, abstract = {

Photosynthetic pigments and several structural characteristics were measured in leaves of Andropogon gerardii from tallgrass prairie populations in an unburned, low-irradiance site and a burned, high-irradiance site to determine if these species displayed sun/shade differences similar to those documented in forest species. Early in the growing season, leaves of A. gerardii in the low-irradiance, unburned site had significantly lower stomatal density, pore length, and conductance, as well as specific leaf mass and thickness than leaves from the high-irradiance, burned site. Moreover, the chlorophnyll a:b ratio, carotenoid content, and bundle sheath-vascular complex area were significantly lower, but chlorophyll content (mass/mass) was greater in leaves in unburned vs burned sites. These differences are consistent with sun/shade adaptations reported for forest understory plants and may contribute to the low productivity of A. gerardii in unburned tallgrass prairie

}, keywords = {LTER-KNZ, tallgrass prairie}, doi = {http://www.jstor.org/stable/2443720}, author = {Alan K. Knapp and Gilliam, F.S.} }