02601nas a2200265 4500008004100000245007800041210006900119300001600188490000800204520179500212100002002007700001702027700003002044700002102074700002702095700002002122700002102142700001402163700002102177700002302198700001802221700001402239700002302253856005902276 2020 eng d00aResolving the Dust Bowl paradox of grassland responses to extreme drought0 aResolving the Dust Bowl paradox of grassland responses to extrem a22249-222550 v1173 a

During the 1930s Dust Bowl drought in the central United States, species with the C3 photosynthetic pathway expanded throughout C4-dominated grasslands. This widespread increase in C3 grasses during a decade of low rainfall and high temperatures is inconsistent with well-known traits of C3 vs. C4 pathways. Indeed, water use efficiency is generally lower, and photosynthesis is more sensitive to high temperatures in C3 than C4 species, consistent with the predominant distribution of C3 grasslands in cooler environments and at higher latitudes globally. We experimentally imposed extreme drought for 4 y in mixed C3/C4 grasslands in Kansas and Wyoming and, similar to Dust Bowl observations, also documented three- to fivefold increases in C3/C4 biomass ratios. To explain these paradoxical responses, we first analyzed long-term climate records to show that under nominal conditions in the central United States, C4 grasses dominate where precipitation and air temperature are strongly related (warmest months are wettest months). In contrast, C3 grasses flourish where precipitation inputs are less strongly coupled to warm temperatures. We then show that during extreme drought years, precipitation–temperature relationships weaken, and the proportion of precipitation falling during cooler months increases. This shift in precipitation seasonality provides a mechanism for C3 grasses to respond positively to multiyear drought, resolving the Dust Bowl paradox. Grasslands are globally important biomes and increasingly vulnerable to direct effects of climate extremes. Our findings highlight how extreme drought can indirectly alter precipitation seasonality and shift ecosystem phenology, affecting function in ways not predictable from key traits of C3 and C4 species.

1 aKnapp, Alan, K.1 aChen, Anping1 aGriffin-Nolan, Robert, J.1 aBaur, Lauren, E.1 aCarroll, Charles, J.W.1 aGray, Jesse, E.1 aHoffman, Ava, M.1 aLi, Xiran1 aPost, Alison, K.1 aSlette, Ingrid, J.1 aCollins, S, L1 aLuo, Yiqi1 aSmith, Melinda, D. uhttp://www.pnas.org/lookup/doi/10.1073/pnas.192203011702738nas a2200205 4500008004100000245006300041210006200104300001600166490000700182520212900189100002302318700002102341700001402362700001702376700002202393700001902415700001602434700002002450856006202470 2019 eng d00aHow ecologists define drought, and why we should do better0 aHow ecologists define drought and why we should do better a3193 - 32000 v253 a

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.

1 aSlette, Ingrid, J.1 aPost, Alison, K.1 aAwad, Mai1 aEven, Trevor1 aPunzalan, Arianna1 aWilliams, Sere1 aSmith, M.D.1 aKnapp, Alan, K. uhttps://onlinelibrary.wiley.com/doi/abs/10.1111/gcb.1474703181nas a2200289 4500008004100000245012300041210006900164300001600233490000700249520223100256100003002487700002202517700002802539700002002567700001802587700002102605700002102626700002102647700002302668700002002691700002102711700002402732700002202756700002002778700001702798856007602815 2018 eng d00aTrait selection and community weighting are key to understanding ecosystem responses to changing precipitation regimes0 aTrait selection and community weighting are key to understanding a1746 - 17560 v323 a

1. Plant traits can be used to predict ecosystem responses to environmental change using a response–effect trait framework. To do this, appropriate traits must be identified that explain a species' influence on ecosystem function (“effect traits”) and the response of those species to environmental change (“response traits”). Response traits are often identified and measured along gradients in plant resources, such as water availability; however, precipitation explains very little variation in most plant traits globally. Given the strong relationship between plant traits and ecosystem functions, such as net primary productivity (NPP), and between NPP and precipitation, the lack of correlation between precipitation and plant traits is surprising.

2. We address this issue through a systematic review of >500 published studies that describe plant trait responses to altered water availability. The overarching goal of this review was to identify potential causes for the weak relationship between commonly measured plant traits and water availability so that we may identify more appropriate “response traits.”

3. We attribute weak trait–precipitation relationships to an improper selection of traits (e.g., nonhydraulic traits) and a lack of trait‐based approaches that adjust for trait variation within communities (only 4% of studies measure community‐weighted traits). We then highlight the mechanistic value of hydraulic traits as more appropriate “response traits” with regard to precipitation, which should be included in future community‐scale trait surveys.

4. Trait‐based ecology has the potential to improve predictions of ecosystem responses to predicted changes in precipitation; however, this predictive power depends heavily on the identification of reliable response and effect traits. To this end, trait surveys could be improved by a selection of traits that reflect physiological functions directly related to water availability with traits weighted by species relative abundance.

1 aGriffin-Nolan, Robert, J.1 aBushey, Julie, A.1 aCarroll, Charles, J. W.1 aChallis, Anthea1 aChieppa, Jeff1 aGarbowski, Magda1 aHoffman, Ava, M.1 aPost, Alison, K.1 aSlette, Ingrid, J.1 aSpitzer, Daniel1 aZambonini, Dario1 aOcheltree, Troy, W.1 aTissue, David, T.1 aKnapp, Alan, K.1 aFox, Charles uhttps://besjournals.onlinelibrary.wiley.com/doi/10.1111/1365-2435.13135