03509nas a2200313 4500008004100000245008500041210006900126300001600195490000800211520253200219653000902751653001902760653003002779653001202809653002502821653002802846100003002874700002102904700002402925700002402949700002102973700002302994700002403017700001603041700002503057700002003082700001703102856007603119 2019 eng d00aShifts in plant functional composition following long-term drought in grasslands0 aShifts in plant functional composition following longterm drough a2133 - 21480 v1073 a

 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.

10aANPP10aClimate change10acommunity weighted traits10aDrought10aFunctional diversity10aplant functional traits1 aGriffin-Nolan, Robert, J.1 aBlumenthal, D.M.1 aCollins, Scott., L.1 aFarkas, Timothy, E.1 aHoffman, Ava, M.1 aMueller, Kevin, E.1 aOcheltree, Troy, W.1 aSmith, M.D.1 aWhitney, Kenneth, D.1 aKnapp, Alan, K.1 aJones, Holly uhttps://besjournals.onlinelibrary.wiley.com/doi/10.1111/1365-2745.1325202697nas a2200193 4500008004100000245009100041210006900132300001000201520208500211653002102296653001202317653002302329653001302352653002102365100001702386700002002403700001602423856006402439 2018 eng d00aCarbon exchange responses of a mesic grassland to an extreme gradient of precipitation0 aCarbon exchange responses of a mesic grassland to an extreme gra a1 -123 a

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.

10aClimate extremes10aDrought10aPrimary production10aRainfall10asoil respiration1 aFelton, A.J.1 aKnapp, Alan, K.1 aSmith, M.D. uhttps://link.springer.com/article/10.1007/s00442-018-4284-202051nas a2200205 4500008004100000245011100041210006900152300001200221490000800233520138900241653002301630653002101653653001201674653001801686653002201704100001701726700002001743700001601763856006601779 2017 eng d00aPhotosynthetic responses of a dominant C4 grass to an experimental heat wave are mediated by soil moisture0 aPhotosynthetic responses of a dominant C4 grass to an experiment a303-3130 v1833 a

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.

10aCanopy temperature10aClimate extremes10aDrought10aEcophysiology10atallgrass prairie1 aHoover, D.L.1 aKnapp, Alan, K.1 aSmith, M.D. uhttps://link.springer.com/article/10.1007%2Fs00442-016-3755-600681nas a2200241 4500008004100000245006800041210006800109300001200177490000700189653001200196653001400208653001200222653001400234653002500248653001300273100002000286700002000306700001400326700001600340700001700356700001700373856004900390 2016 eng d00aRangeland responses to predicted increases in drought extremity0 aRangeland responses to predicted increases in drought extremity a191-1960 v3810aDrought10agrassland10asavanna10ashrubland10astate and transition10awoodland1 aBreshears, D.D.1 aKnapp, Alan, K.1 aLaw, D.J.1 aSmith, M.D.1 aTwidwell, D.1 aWonkka, C.L. uhttp://dx.doi.org/10.1016/j.rala.2016.06.00902231nas a2200349 4500008004100000245008200041210006900123300001300192490000700205520128800212653001901500653001201519653001501531653001801546653001501564100002001579700001401599700001701613700001801630700001201648700001901660700001601679700001601695700001501711700001401726700001801740700001701758700001501775700001401790700001301804856006401817 2008 eng d00aConsequences of more extreme precipitation regimes for terrestrial ecosystems0 aConsequences of more extreme precipitation regimes for terrestri a811 -8210 v583 a

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.

10aClimate change10aDrought10aEcosystems10aPrecipitation10asoil water1 aKnapp, Alan, K.1 aBeier, C.1 aBriske, D.D.1 aClassen, A.T.1 aLuo, Y.1 aReichstein, M.1 aSmith, M.D.1 aSmith, S.D.1 aBell, J.E.1 aFay, P.A.1 aHeisler, J.L.1 aLeavitt, S.W1 aSherry, R.1 aSmith, B.1 aWeng, E. uhttps://academic.oup.com/bioscience/article/58/9/811/250853