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Extreme droughts generally decrease productivity in grassland ecosystems1,2,3 with negative consequences for nature’s contribution to people4,5,6,7. The extent to which this negative effect varies among grassland types and over time in response to multi-year extreme drought remains unclear. Here, using a coordinated distributed experiment that simulated four years of growing-season drought (around 66% rainfall reduction), we compared drought sensitivity within and among six representative grasslands spanning broad precipitation gradients in each of Eurasia and North America—two of the Northern Hemisphere’s largest grass-dominated regions. Aboveground plant production declined substantially with drought in the Eurasian grasslands and the effects accumulated over time, while the declines were less severe and more muted over time in the North American grasslands. Drought effects on species richness shifted from positive to negative in Eurasia, but from negative to positive in North America over time. The differing responses of plant production in these grasslands were accompanied by less common (subordinate) plant species declining in Eurasian grasslands but increasing in North American grasslands. Our findings demonstrate the high production sensitivity of Eurasian compared with North American grasslands to extreme drought (43.6% versus 25.2% reduction), and the key role of subordinate species in determining impacts of extreme drought on grassland productivity. 

Summary A prevailing hypothesis posits that achieving higher maximum rates of leaf carbon gain and water loss is constrained by geometry and/or selection to limit the allocation of epidermal area to stomata (f_S). Under this ‘stomatal‐area minimization hypothesis’, higherg_s,maxis associated with greater numbers of smaller stomata because this trait combination increasesg_s,maxwith minimal increase inf_S, leading to relative conservation off_Ssemi‐independent ofg_s,maxdue to coordination in stomatal size, density, and pore depth. An alternative hypothesis is that the evolution of higherg_s,maxcan be enabled by a greater epidermal area allocated to stomata, leading to positive covariation betweenf_Sandg_s,max; we call this the ‘stomatal‐area adaptation hypothesis’. Under this hypothesis, the interspecific scaling betweeng_s,max, stomatal density, and stomatal size is a by‐product of selection on a moving optimalg_s,max.We integrated biophysical and evolutionary quantitative genetic modeling with phylogenetic comparative analyses of a global data set of stomatal density and size from 2408 vascular forest species. The models present specific assumptions of both hypotheses and deduce predictions that can be evaluated with our empirical analyses of forest plants.There are three main results. First, neither the stomatal‐area minimization nor adaptation hypothesis is sufficient to be supported. Second, estimates of interspecific scaling from common regression methods cannot reliably distinguish between hypotheses when stomatal size is bounded. Third, we reconcile both hypotheses with the data by including an additional assumption that stomatal size is bounded by a wide range and under selection; we refer to this synthetic hypothesis as the ‘stomatal adaptation + bounded size’ hypothesis.This study advances our understanding of scaling between stomatal size and density by mathematically describing specific assumptions of competing hypotheses, demonstrating that existing hypotheses are inconsistent with observations, and reconciling these hypotheses with phylogenetic comparative analyses by postulating a synthetic model of selection ong_s,max,f_S, and stomatal size. 

Abstract Plant traits can be helpful for understanding grassland ecosystem responses to climate extremes, such as severe drought. However, intercontinental comparisons of how drought affects plant functional traits and ecosystem functioning are rare. The Extreme Drought in Grasslands experiment (EDGE) was established across the major grassland types in East Asia and North America (six sites on each continent) to measure variability in grassland ecosystem sensitivity to extreme, prolonged drought. At all sites, we quantified community‐weighted mean functional composition and functional diversity of two leaf economic traits, specific leaf area and leaf nitrogen content, in response to drought. We found that experimental drought significantly increased community‐weighted means of specific leaf area and leaf nitrogen content at all North American sites and at the wetter East Asian sites, but drought decreased community‐weighted means of these traits at moderate to dry East Asian sites. Drought significantly decreased functional richness but increased functional evenness and dispersion at most East Asian and North American sites. Ecosystem drought sensitivity (percentage reduction in aboveground net primary productivity) positively correlated with community‐weighted means of specific leaf area and leaf nitrogen content and negatively correlated with functional diversity (i.e., richness) on an intercontinental scale, but results differed within regions. These findings highlight both broad generalities but also unique responses to drought of community‐weighted trait means as well as their functional diversity across grassland ecosystems. 

Abstract Plant traits are useful proxies of plant strategies and can influence community and ecosystem responses to climate extremes, such as severe drought. Few studies, however, have investigated both the immediate and lagged effects of drought on community‐weighted mean (CWM) plant traits, with even less research on the relative roles of interspecific vs. intraspecific trait variability in such responses.We experimentally reduced growing season precipitation by 66% in two cold‐semi‐arid grassland sites in northern China for four consecutive years to explore the drought resistance of CWM traits as well as their recovery 2 years following the drought. In addition, we isolated the effects of both interspecific and intraspecific trait variability on shifts in CWM traits.At both sites, we observed significant effects of drought on interspecific and intraspecific trait variability which, in some cases, led to significant changes in CWM traits. For example, drought led to reduced CWM plant height and leaf phosphorous content, but increased leaf carbon content at both sites, with responses primarily due to intraspecific trait shifts. Surprisingly, these CWM traits recovered completely 2 years after the extreme drought. Intraspecific trait variability influenced CWM traits via both positive and negative covariation with interspecific trait variability during drought and recovery phases.These findings highlight the important role of interspecific and intraspecific trait variability in driving the response and recovery of CWM traits following extreme, prolonged drought. Read the freePlain Language Summaryfor this article on the Journal blog. 

Abstract Seeds provide the basis of genetic diversity in perennial grassland communities and their traits may influence ecosystem resistance to extreme drought. However, we know little about how drought effects the community functional composition of seed traits and the corresponding implications for ecosystem resistance to drought.We experimentally removed 66% of growing season precipitation for 4 years across five arid and semi‐arid grasslands in northern China and assessed how this multi‐year drought impacted community‐weighted means (CWMs) of seed traits, seed trait functional diversity and above‐ground net primary productivity (ANPP).Experimental drought had limited effects on CWM traits and the few effects that did occur varied by site and year. For three separate sites, and in different years, drought reduced seed length and phosphorus content but increased both seed and seed‐coat thickness. Additionally, drought led to increased seed functional evenness, divergence, dispersion and richness, but only in some sites, and mostly in later years following cumulative effects of water limitation. However, we observed a strong negative relationship between drought‐induced reductions in ANPP and CWMs of seed‐coat thickness, indicating that a high abundance of dominant species with thick seeds may increase ecosystem resistance to drought. Seed trait functional diversity was not significantly predictive of ANPP, providing little evidence for a diversity effect.Our results suggest that monitoring community composition with a focus on seed traits may provide a valuable indicator of ecosystem resistance to future droughts despite inconsistent responses of seed trait composition overall. This highlights the importance of developing a comprehensive seed and reproductive traits database for arid and semi‐arid grassland biomes. Read the freePlain Language Summaryfor this article on the Journal blog. 

Abstract Intertidal sands are global hotspots of terrestrial and marine carbon cycling with strong hydrodynamic forcing by waves and tides and high macrofaunal activity. Yet, the relative importance of hydrodynamics and macrofauna in controlling these ecosystems remains unclear. Here, we compare geochemical gradients and bacterial, archaeal, and eukaryotic gene sequences in intertidal sands dominated by subsurface deposit-feeding worms (Abarenicola pacifica) to adjacent worm-free areas. We show that hydrodynamic forcing controls organismal assemblages in surface sediments, while in deeper layers selective feeding by worms on fine, algae-rich particles strongly decreases the abundance and richness of all three domains. In these deeper layers, bacterial and eukaryotic network connectivity decreases, while percentages of clades involved in degradation of refractory organic matter, oxidative nitrogen, and sulfur cycling increase. Our findings reveal macrofaunal activity as the key driver of biological community structure and functioning, that in turn influence carbon cycling in intertidal sands below the mainly physically controlled surface layer. 

Abstract Asexual reproduction plays a fundamental role in the structure, dynamics and persistence of perennial grasslands. Thus, assessing how asexual reproductive traits of plant communities respond to drought may be key for understanding grassland resistance to drought and recovery following drought.Here, we quantified three asexual reproductive traits (i.e. above‐ground tiller abundance, below‐ground bud abundance and the ratio of tillers to buds) during a 4‐year severe drought and a 2‐year drought recovery period in four grasslands that spanned an aridity gradient in northern China. We also assessed the relationship between these traits and the resistance and recovery of above‐ground net primary productivity (ANPP).We found that drought had limited and largely inconsistent effects on asexual reproduction among drought and recovery years and grasslands overall. Drought increased tiller abundance in the first treatment year and reduced bud banks by the fourth treatment year across grasslands. However, neither of the three asexual reproductive traits were correlated with drought resistance of ANPP. Drought legacies differed among the four grasslands with positive, negative and no legacies evident for the three asexual reproductive traits, and no clear relationship with aridity. Bud banks and tiller to bud ratio decreased and increased, respectively, in the first recovery year, but not in the second recovery year. In contrast to drought resistance, community bud abundance was strongly related to recovery, such that communities with higher bud abundance had greater ANPP recovery following drought.Synthesis. These results suggest that asexual reproductive traits may be important drivers of ecosystem recovery after drought, but that variable responses of these asexual reproduction traits during drought complicates predictions of overall grassland responses.