03239nas 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.1350403284nas a2200385 4500008004100000245011800041210007100159300001400230490000700244520218900251100001702440700001402457700001802471700001802489700001402507700001702521700001402538700001702552700001802569700001402587700002102601700001702622700002302639700001902662700001402681700001902695700001602714700001502730700001802745700001802763700002002781700001602801700001902817856006202836 2016 eng d00aFew multiyear precipitation–reduction experiments find a shift in the productivity–precipitation relationship0 aFew multiyear precipitation–reduction experiments find a shift i a2570-25810 v223 a

Well-defined productivity–precipitation relationships of ecosystems are needed as benchmarks for the validation of land models used for future projections. The productivity–precipitation relationship may be studied in two ways: the spatial approach relates differences in productivity to those in precipitation among sites along a precipitation gradient (the spatial fit, with a steeper slope); the temporal approach relates interannual productivity changes to variation in precipitation within sites (the temporal fits, with flatter slopes). Precipitation–reduction experiments in natural ecosystems represent a complement to the fits, because they can reduce precipitation below the natural range and are thus well suited to study potential effects of climate drying. Here, we analyse the effects of dry treatments in eleven multiyear precipitation–manipulation experiments, focusing on changes in the temporal fit. We expected that structural changes in the dry treatments would occur in some experiments, thereby reducing the intercept of the temporal fit and displacing the productivity–precipitation relationship downward the spatial fit. The majority of experiments (72%) showed that dry treatments did not alter the temporal fit. This implies that current temporal fits are to be preferred over the spatial fit to benchmark land-model projections of productivity under future climate within the precipitation ranges covered by the experiments. Moreover, in two experiments, the intercept of the temporal fit unexpectedly increased due to mechanisms that reduced either water loss or nutrient loss. The expected decrease of the intercept was observed in only one experiment, and only when distinguishing between the late and the early phases of the experiment. This implies that we currently do not know at which precipitation–reduction level or at which experimental duration structural changes will start to alter ecosystem productivity. Our study highlights the need for experiments with multiple, including more extreme, dry treatments, to identify the precipitation boundaries within which the current temporal fits remain valid.

1 aEstiarte, M.1 aVicca, S.1 aPeñuelas, J.1 aBahn, Michael1 aBeier, C.1 aEmmett, B.A.1 aFay, P.A.1 aHanson, P.J.1 aHasibeder, R.1 aKigel, J.1 aKröel-Dulay, G.1 aLarsen, K.S.1 aLellei-Kovács, E.1 aLimousin, J-M.1 aOgaya, R.1 aOurcival, J-M.1 aReinsch, S.1 aSala, O.E.1 aSchmidt, I.K.1 aSternberg, M.1 aTielbörger, K.1 aTietema, A.1 aJanssens, I.A. uhttps://onlinelibrary.wiley.com/doi/abs/10.1111/gcb.1326902231nas 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/58/9/811/25085302905nas a2200313 4500008004100000245013800041210006900179300001500248490000700263520198500270100001202255700001502267700001702282700001702299700001302316700001302329700001502342700001402357700001402371700001502385700001602400700001502416700001702431700002002448700001502468700001602483700001502499856007702514 2008 eng d00aModeled interactive effects of precipitation, temperature, and CO2 on ecosystem carbon and water dynamics in different climatic zones0 aModeled interactive effects of precipitation temperature and CO2 a1986 -19990 v143 a

Interactive effects of multiple global change factors on ecosystem processes are complex. It is relatively expensive to explore those interactions in manipulative experiments. We conducted a modeling analysis to identify potentially important interactions and to stimulate hypothesis formulation for experimental research. Four models were used to quantify interactive effects of climate warming (T), altered precipitation amounts [doubled (DP) and halved (HP)] and seasonality (SP, moving precipitation in July and August to January and February to create summer drought), and elevated [CO2] (C) on net primary production (NPP), heterotrophic respiration (Rh), net ecosystem production (NEP), transpiration, and runoff. We examined those responses in seven ecosystems, including forests, grasslands, and heathlands in different climate zones. The modeling analysis showed that none of the three-way interactions among T, C, and altered precipitation was substantial for either carbon or water processes, nor consistent among the seven ecosystems. However, two-way interactive effects on NPP, Rh, and NEP were generally positive (i.e. amplification of one factor's effect by the other factor) between T and C or between T and DP. A negative interaction (i.e. depression of one factor's effect by the other factor) occurred for simulated NPP between T and HP. The interactive effects on runoff were positive between T and HP. Four pairs of two-way interactive effects on plant transpiration were positive and two pairs negative. In addition, wet sites generally had smaller relative changes in NPP, Rh, runoff, and transpiration but larger absolute changes in NEP than dry sites in response to the treatments. The modeling results suggest new hypotheses to be tested in multifactor global change experiments. Likewise, more experimental evidence is needed for the further improvement of ecosystem models in order to adequately simulate complex interactive processes.

1 aLuo, Y.1 aGerten, D.1 aLe Maire, G.1 aParton, W.J.1 aWeng, E.1 aZhou, X.1 aKeough, C.1 aBeier, C.1 aCiais, P.1 aCramer, W.1 aDukes, J.S.1 aEmmett, B.1 aHanson, P.J.1 aKnapp, Alan, K.1 aLinder, S.1 aNepstad, D.1 aRustad, L. uhttps://onlinelibrary.wiley.com/doi/abs/10.1111/j.1365-2486.2008.01629.x02411nas a2200313 4500008004100000245010500041210006900146300001000215490000700225520152600232100001501758700001201773700001701785700001701802700001501819700001301834700001401847700001401861700001501875700001601890700001501906700001701921700002001938700001501958700001601973700001501989700001602004856007702020 2008 eng d00aModelled effects of precipitation on ecosystem carbon and water dynamics in different climatic zones0 aModelled effects of precipitation on ecosystem carbon and water  a1 -150 v143 a

The ongoing changes in the global climate expose the world's ecosystems not only to increasing CO2 concentrations and temperatures but also to altered precipitation (P) regimes. Using four well-established process-based ecosystem models (LPJ, DayCent, ORCHIDEE, TECO), we explored effects of potential P changes on water limitation and net primary production (NPP) in seven terrestrial ecosystems with distinctive vegetation types in different hydroclimatic zones. We found that NPP responses to P changes differed not only among sites but also within a year at a given site. The magnitudes of NPP change were basically determined by the degree of ecosystem water limitation, which was quantified here using the ratio between atmospheric transpirational demand and soil water supply. Humid sites and/or periods were least responsive to any change in P as compared with moderately humid or dry sites/periods. We also found that NPP responded more strongly to doubling or halving of P amount and a seasonal shift in P occurrence than that to altered P frequency and intensity at constant annual amounts. The findings were highly robust across the four models especially in terms of the direction of changes and largely consistent with earlier P manipulation experiments and modelling results. Overall, this study underscores the widespread importance of P as a driver of change in ecosystems, although the ultimate response of a particular site will depend on the detailed nature and seasonal timing of P change.

1 aGerten, D.1 aLuo, Y.1 aLe Maire, G.1 aParton, W.J.1 aKeough, C.1 aWeng, E.1 aBeier, C.1 aCiais, P.1 aCramer, W.1 aDukes, J.S.1 aEmmett, B.1 aHanson, P.J.1 aKnapp, Alan, K.1 aLinder, S.1 aNepstad, D.1 aRustad, L.1 aSowerby, A. uhttps://onlinelibrary.wiley.com/doi/abs/10.1111/j.1365-2486.2008.01651.x