1. 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 inter- vs. intraspecific trait variability in such responses. 2. We experimentally reduced growing season precipitation by 66% in two cold-semiarid grassland sites in northern China for four consecutive years to explore the drought resistance of CWM traits as well as their recovery two years following the drought. Additionally, we isolated the effects of both inter- and intraspecific trait variability on shifts in CWM traits. 3. At both sites, we observed significant effects of drought on inter- 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 two 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. 4. These findings highlight the important role of inter- and intraspecific trait variability in driving the response and recovery of CWM traits following extreme, prolonged drought.
1. 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. 2. We experimentally removed 66% of growing season precipitation for four years across five arid and semiarid grasslands in northern China and assessed how this multi-year drought impacted community-weighted means (CWM) of seed traits, seed trait functional diversity, and aboveground net primary productivity (ANPP). 3. 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. 4. 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 semiarid grassland biomes.
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 Questions Plant traits can predict a species‘ relative abundance and its influence on ecosystem processes. However, trait expression and the relative abundance of a species are also influenced by its abiotic and biotic environment. Here, we ask whether the relationship between plant traits and relative abundance is modified by abiotic (e.g., climate and topography) and biotic factors (e.g., community taxonomic and functional diversity) across Californian grasslands. Location San Francisco Bay Area. Methods We measured specific leaf area (SLA; leaf area/dry mass) and plant height of 19 grass species (family: Poaceae) across 117 plots. We also quantified the relative abundance of each species as well as several biotic attributes of the neighboring grass community including total plant cover, species richness and evenness, community‐weighted mean (CWM) traits, and functional diversity. Using multiple linear regression, we assessed whether abundance could be predicted from traits and the interactions between traits and both biotic and abiotic factors. We fit similar models predicting traits from relative abundance. Results Grass species had higher relative abundance in plots where they were taller and had higher SLA. They were also more abundant in communities with low functional richness (FRic) and high functional evenness (FEve), perhaps because of the low resource‐use efficiency of their neighbors and a lack of dominant grasses. Neither abundance nor plant height were associated with abiotic variables, although SLA responded predictably to precipitation according to a bell‐shaped curve. Grasses were taller where they were more abundant, but the impact of abundance on SLA depended on community FEve. Finally, we show strong evidence for community trait similarity, whereby an individual‘s trait expression was positively correlated with the traits of its grass neighbors. Conclusions Taken together, these results imply that traits are predictive of abundance and vice versa, and these relationships depend on biotic interactions more than climate.
Abstract Plant populations are limited by resource availability and exhibit physiological trade‐offs in resource acquisition strategies. These trade‐offs may constrain the ability of populations to exhibit fast growth rates under water limitation and high cover of neighbours. However, traits that confer drought tolerance may also confer resistance to competition. It remains unclear how fitness responses to these abiotic conditions and biotic interactions combine to structure grassland communities and how this relationship may change along a gradient of water availability. To address these knowledge gaps, we estimated the low‐density growth rates of populations in drought conditions with low neighbour cover and in ambient conditions with average neighbour cover for 82 species in six grassland communities across the Central Plains and Southwestern United States. We assessed the relationship between population tolerance to drought and resistance to competition and determined if this relationship was consistent across a precipitation gradient. We also tested whether population growth rates could be predicted using plant functional traits. Across six sites, we observed a positive correlation between low‐density population growth rates in drought and in the presence of interspecific neighbours. This positive relationship was particularly strong in the grasslands of the northern Great Plains but weak in the most xeric grasslands. High leaf dry matter content and a low (more negative) leaf turgor loss point were associated with high population growth rates in drought and with neighbours in most grassland communities. Synthesis : A better understanding of how both biotic and abiotic factors impact population fitness provides valuable insights into how grasslands will respond to extreme drought. Our results advance plant strategy theory by suggesting that drought tolerance increases population resistance to interspecific competition in grassland communities. However, this relationship is not evident in the driest grasslands, where above‐ground competition is likely less important. Leaf dry matter content and turgor loss point may help predict which populations will establish and persist based on local water availability and neighbour cover, and these predictions can be used to guide the conservation and restoration of biodiversity in grasslands.
Abstract Clonal demographic traits play important roles in regulating community dynamics. Yet, it remains unclear how the responses of these clonal traits to drought might depend on previous drought exposure, and how drought responses vary among grasslands. We conducted a repeated drought experiment across four grasslands along an aridity gradient in northern China. We assessed the effects of single (precipitation reduction in 2021–2022) versus recurrent (precipitation reduction in 2015–2018 and 2021–2022) drought on bud density, shoot density, and the ratio of bud to shoot density. Drought reduced bud density at all grasslands and shoot density at most grasslands. Drought reduced the ratio of bud‐to‐shoot density only in the most arid grasslands. Recurrent drought had larger negative effects than a single drought on bud density and composition of bud and shoot at only one of four grasslands, and on shoot density at two of four grasslands. Our results suggest that previous drought exposure can alter the response of plant clonal demographic traits to subsequent drought in some but not all grasslands, and that responses can vary with mean climate.
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