María Grisel Longo

Understanding the drivers of plant community stability is crucial for predicting ecosystem responses to extreme drought events. In grasslands, drought resistance supports the maintenance of key functions such as above‐ground primary productivity, making the identification of resistance drivers essential to guide management under climate change. Proposed factors contributing to grassland stability include multiple diversity facets, functional traits and long‐term climate, but most assessments focus on temporal invariability under historical disturbance regimes, leaving mechanisms of extreme drought resistance and their variation across climatic contexts relatively underexplored. Here, we analysed data from 54 grassland sites of the International Drought Experiment to examine the resistance of above‐ground net primary productivity to a short‐term (i.e. 1 year) extreme drought. We investigated the relative importance and joint influence of functional composition (i.e. community‐weighted means of leaf and root traits), plant diversity facets (taxonomic, functional and phylogenetic) and climate (aridity and rainfall variability) on drought resistance. We used structural equation models to disentangle direct, indirect, and moderating pathways linking these drivers to drought resistance. Long‐term aridity appeared as one of the most important drivers of grassland resistance to drought, with more arid sites showing lower resistance. Moreover, aridity impacted resistance through indirect effects by shaping functional composition and plant diversity, and by moderating the influence of plant diversity and functional composition. Functional composition related to dehydration avoidance and dehydration tolerance was also positively associated with resistance, while diversity had a weaker relationship with resistance, mostly through functional and phylogenetic facets. Interannual rainfall variability also influenced resistance, with different effects in more arid versus humid and less arid sites. Synthesis . Widely studied stability drivers such as plant diversity and functional composition have only partial explanatory power for short‐term drought resistance of above‐ground productivity in grasslands at a global scale. The abiotic context, particularly long‐term aridity, is crucial for understanding ecosystem responses to rainfall variation and can improve predictive models for advancing the study of ecosystem resistance to drought. Along with management practices that target high species diversity or specific traits, restoration and conservation practices should support vulnerable sites experiencing high aridity

Functional group composition can positively influence plant community invasibility through niche vacancy when the resident community does not sufficiently overlap with the invaders. Phenology plays a key role in determining functional group composition, as it defines the temporal scale on which niche overlap occurs. Defoliation may regulate the effect of functional group composition by releasing resources, resulting in interactive effects. In a manipulative experiment, we assembled native communities with different functional group compositions: perennial warm- and cool-season grasses growing either separately or together. These communities were subjected to two defoliation regimes to test the hypothesis that niche overlap determines the invasion success of Lolium multiflorum, an annual grass belonging to a functional group under-represented in the native flora of the region. We also hypothesized that higher defoliation intensity would increase community invasibility, promoting invasion and reducing the differences observed between communities under lower defoliation intensity. We evaluated this in two ontogenetic stages of the invader by counting seedlings and measuring above-ground biomass at the end of the growing season. The evidence shows that community invasibility is mediated by functional group composition through phenological overlap, although priority effects were also important. Defoliation intensity promoted seedling establishment similarly across all communities but did not affect the resulting above-ground biomass at the end of the growing season. Instead, it negatively impacted the performance of the invader growing in monoculture. Considering the variable effects on each ontogenetic stage is important for developing targeted management strategies at the appropriate temporal scale.

Climate change is increasing the frequency and severity of short-term (~1 y) drought events—the most common duration of drought—globally. Yet the impact of this intensification of drought on ecosystem functioning remains poorly resolved. This is due in part to the widely disparate approaches ecologists have employed to study drought, variation in the severity and duration of drought studied, and differences among ecosystems in vegetation, edaphic and climatic attributes that can mediate drought impacts. To overcome these problems and better identify the factors that modulate drought responses, we used a coordinated distributed experiment to quantify the impact of short-term drought on grassland and shrubland ecosystems. With a standardized approach, we imposed ~a single year of drought at 100 sites on six continents. Here we show that loss of a foundational ecosystem function—aboveground net primary production (ANPP)—was 60% greater at sites that experienced statistically extreme drought (1-in-100-y event) vs. those sites where drought was nominal (historically more common) in magnitude (35% vs. 21%, respectively). This reduction in a key carbon cycle process with a single year of extreme drought greatly exceeds previously reported losses for grasslands and shrublands. Our global experiment also revealed high variability in drought response but that relative reductions in ANPP were greater in drier ecosystems and those with fewer plant species. Overall, our results demonstrate with unprecedented rigor that the global impacts of projected increases in drought severity have been significantly underestimated and that drier and less diverse sites are likely to be most vulnerable to extreme drought.