Which, when, and where traits matter: functional trait-mediated species associations shift with microenvironmental and climatic variation.

Predicting coexistence in species-rich plant communities requires understanding the role of functional traits in species interactions and the stability of these associations to environmental variation. In a manipulative factorial field experiment in an annual community (> 45 500 individuals and 45 species), we followed plant associations from seedling to adult on both preserved and altered biological soil crust (BSC) over 2 yr with contrasting weather conditions. Neighborhood models quantified how among-species differences in key functional traits affected coexistence and how these effects varied in response to ontogenetic stage, environmental variation, and spatial scale. Trait-based models revealed a prevalent negative relationship between functional differences and species associations, but these relationships varied predictively with ontogenetic stage, environment, and spatial scale. Functional dissimilarity in traits, such as specific leaf area, seed mass, and reproductive-to-vegetative biomass ratio consistently mediated species segregation, especially during wet years and among seedlings, while BSC disturbance altered trait-mediated associations differently at fine and coarse spatial scales. Trait-mediated species associations shift predictably with environmental variation, suggesting that coexistence reflects a dynamic balance between environmental filtering and competitive interactions. This work provides novel experimental evidence of when, where, and which traits predict community assembly, which has implications for forecasting biodiversity responses to global environmental change.