02417nas a2200157 4500008004100000245014700041210006900188300001100257490000600268520185900274100001702133700001702150700001802167700001602185856005802201 2023 eng d00aRepeated extreme droughts decrease root production, but not the potential for post‐drought recovery of root production, in a mesic grassland0 aRepeated extreme droughts decrease root production but not the p ae088990 v13 a

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'), or years 5–6 (‘Drought 2') 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.

1 aSlette, I.J.1 aHoover, D.L.1 aSmith, M., D.1 aKnapp, A.K. uhttps://onlinelibrary.wiley.com/doi/10.1111/oik.0889902541nas a2200145 4500008004100000245011100041210006900152300001400221490000800235520202700243100002402270700001702294700002002311856006402331 2021 eng d00aDeconstructing precipitation variability: rainfall event size and timing uniquely alter ecosystem dynamics0 aDeconstructing precipitation variability rainfall event size and a3356-33690 v1093 a1 aGriffin-Nolan, R.J.1 aSlette, I.J.1 aKnapp, Alan, K. uhttps://onlinelibrary.wiley.com/doi/10.1111/1365-2745.1372400745nas a2200241 4500008004100000245009500041210006900136100002000205700001700225700002400242700001500266700001800281700001700299700001500316700001500331700001900346700001100365700001800376700001200394700001600406700001600422856006500438 2021 eng d00aIs a drought a drought in grasslands? Productivity responses to different types of drought0 adrought a drought in grasslands Productivity responses to differ1 aCarroll, C.J.W.1 aSlette, I.J.1 aGriffin-Nolan, R.J.1 aBaur, L.E.1 aHoffman, A.M.1 aDenton, E.M.1 aGray, J.E.1 aPost, A.K.1 aJohnston, M.K.1 aYu, Q.1 aCollins, S.L.1 aLuo, Y.1 aSmith, M.D.1 aKnapp, A.K. uhttps://link.springer.com/article/10.1007/s00442-020-04793-802286nas a2200145 4500008004100000245011500041210006900156520176300225100001701988700001802005700001402023700001802037700002002055856006502075 2021 eng d00aEffects of compounded precipitation pattern intensification and drought occur belowground in a mesic grassland0 aEffects of compounded precipitation pattern intensification and 3 a

Climate change is altering precipitation regimes globally, with expectations of intensified precipitation patterns (for example, larger but fewer rainfall events) and more frequent and extreme drought. Both aspects of precipitation change can impact ecosystem function individually, but it is more likely that they will occur in combination. In a central US mesic grassland, we imposed an extreme 2-year drought (growing season precipitation reduced by 66%) on plots with a long-term (16-year) history of exposure to either ambient or intensified precipitation patterns (average threefold increase in event size and threefold decrease in event number during the growing season). While this intensified pattern did not alter total precipitation amount, it generally led to ecosystem responses consistent with a drier environment (for example, reduced soil moisture, aboveground net primary production (ANPP), and soil CO2 flux, but little evidence for altered root biomass). Surprisingly, this history of intensified precipitation patterns did not affect the response of ANPP to the subsequent extreme drought. In contrast, previous exposure to intensified precipitation patterns reduced root production and muted soil CO2 flux responses to rainfall events during drought. Reduced root production in plots experiencing compounded precipitation extremes was driven not by the dominant C4 grass species, Andropogon gerardii, but collectively by the subdominant species in the plant community. Overall, our results reveal that compound changes in precipitation patterns and amount affected this grassland in ways that were less apparent (that is, belowground) than responses to either change individually and significantly reduced ecosystem carbon uptake.

1 aSlette, I.J.1 aBlair, J., M.1 aFay, P.A.1 aSmith, M., D.1 aKnapp, Alan, K. uhttps://link.springer.com/article/10.1007/s10021-021-00714-900422nas a2200109 4500008004100000245009200041210006900133490002100202100001700223700002000240856005200260 2021 eng d00aImpacts of compound precipitation extremes on belowground dynamics in a mesic grassland0 aImpacts of compound precipitation extremes on belowground dynami0 vPhD Dissertation1 aSlette, I.J.1 aKnapp, Alan, K. uhttps://mountainscholar.org/handle/10217/23432202332nas a2200265 4500008004100000245008400041210006900125300001400194490000700208520155900215100002001774700002001794700002401814700001701838700001701855700001301872700001701885700001301902700001801915700001801933700001201951700001301963700001601976856007401992 2018 eng d00aA reality check for climate change experiments: Do they reflect the real world?0 areality check for climate change experiments Do they reflect the a2145-21510 v993 a

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.

1 aKnapp, Alan, K.1 aCarroll, C.J.W.1 aGriffin-Nolan, R.J.1 aSlette, I.J.1 aChavez, F.A.1 aBaur, L.1 aFelton, A.J.1 aGray, J.1 aHoffman, A.M.1 aLemoine, N.P.1 aMao, W.1 aPost, A.1 aSmith, M.D. uhttps://esajournals.onlinelibrary.wiley.com/doi/epdf/10.1002/ecy.247403239nas a2200313 4500008004100000245011900041210006900160300001400229490000700243520231400250100002002564700001702584700001402601700002002615700002402635700001602659700001702675700002402692700001702716700001602733700001502749700001902764700001502783700001502798700001702813700001702830700001602847856006202863 2017 eng d00aPushing precipitation to the extremes in distributed experiments: recommendations for simulating wet and dry years0 aPushing precipitation to the extremes in distributed experiments a1774-17820 v233 a

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.

1 aKnapp, Alan, K.1 aAvolio, M.L.1 aBeier, C.1 aCarroll, C.J.W.1 aCollins, Scott., L.1 aDukes, J.S.1 aFraser, L.H.1 aGriffin-Nolan, R.J.1 aHoover, D.L.1 aJentsch, A.1 aLoik, M.E.1 aPhillips, R.P.1 aPost, A.K.1 aSala, O.E.1 aSlette, I.J.1 aYahdjian, L.1 aSmith, M.D. uhttps://onlinelibrary.wiley.com/doi/abs/10.1111/gcb.13504