Environmental variability can structure species coexistence by enhancing niche partitioning. Modern coexistence theory highlights two fluctuation‐dependent temporal coexistence mechanisms —the storage effect and relative nonlinearity – but empirical tests are rare. Here, we experimentally test if environmental fluctuations enhance coexistence in a California annual grassland. We manipulate rainfall timing and relative densities of the grass Avena barbata and forb Erodium botrys, parameterise a demographic model, and partition coexistence mechanisms. Rainfall variability was integral to grass–forb coexistence. Variability enhanced growth rates of both species, and early‐season drought was essential for Erodium persistence. While theoretical developments have focused on the storage effect, it was not critical for coexistence. In comparison, relative nonlinearity strongly stabilised coexistence, where Erodium experienced disproportionately high growth under early‐season drought due to competitive release from Avena. Our results underscore the importance of environmental variability and suggest that relative nonlinearity is a critical if underappreciated coexistence mechanism.

Despite decades of interest, few studies have provided evidence supporting theoretical expectations for coupled relationships between aboveground and belowground diversity and ecosystem functioning in non-manipulated natural ecosystems. We characterized plant species richness and density, soil bacterial, fungal and eukaryotic species richness and phylogenetic diversity (using 16S, ITS, and 18S gene sequencing), and ecosystem function (levels of soil C and N, and rates of microbial enzyme activities) along a natural gradient in plant richness and density in high-elevation, C-deficient soils to examine the coupling between above- and belowground systems. Overall, we observed a strong positive relationship between aboveground (plant richness and density) and belowground (bacteria, fungi, and non-fungal eukaryotes) richness. In addition to the correlations between plants and soil communities, C and N pools, and rates of enzyme activities increased as plant and soil communities became richer and more diverse. Our results suggest that the theoretically expected positive correlation between above- and belowground communities does exist in natural systems, but may be undetectable in late successional ecosystems due to the buildup of legacy organic matter that results in extremely complex belowground communities. In contrast, microbial communities in early successional systems, such as the system described here, are more directly dependent on contemporary inputs from plants and therefore are strongly correlated with plant diversity and density.

Extreme droughts, heat waves, fires, hurricanes, floods, and landslides cause the largest losses in the United States, and globally, from natural hazards linked to weather and climate. There is evidence that the frequency of such extremes is increasing, particularly for heat waves, large fires, and intense precipitation, making better understanding of the probability and consequences of these events imperative. Further, these events are not isolated, but rather interact with each other and with other social and biophysical drivers and conditions, to amplify impacts. Less is known about the nature and strength of these interactions. Natural and social science subfields frame extreme events with different definitions and analytical approaches, often neglecting interactions and the subsequent novel extremes that can arise. Here we propose a framework for social‐environmental extremes, defined as extraordinary events that emerge from interactions among biophysical and social systems. We argue that this definition is critical because it constrains the focus to major events that are capturing societal and scientific attention because of their extreme biophysical drivers and/or the extreme social outcomes. We review how different fields approach extremes as interacting phenomena and propose a synthetic framework that allows analytical separation of the multiple drivers and responses that yield extreme events and extreme effects. We conclude with a future research agenda for understanding the extreme events that matter to society. This agenda will help to identify where, when, and why communities may have high exposure and vulnerability to social‐environmental extremes—informing future mitigation and adaptation strategies.