Species-specific responses to environmental heterogeneity underpin spatial variation in annual plant communities

Spatially heterogeneous environments can promote species coexistence through differences in how species respond to their immediate environments. It remains unclear, however, how species-specific responses to heterogeneous environments contribute to variation in the mechanisms by which community composition promotes coexistence. In this study, we use a reciprocal field experiment to examine how two sources of fine-scale spatial variation-the physical sheltering and soil conditioning separately provided by coarse woody materials (CWM)-influence the composition of winter annual plant communities and the germination and growth performance of three dominant species from these communities. We found that the effects of soil conditioning and physical sheltering from CWM each contributed to spatial variation in plant composition, separately. The presence or absence of CWM in the immediate vicinity of plants explained about 10% of the variation in community composition and had a significant, species-specific effect on germination and biomass for all three focal species. A larger proportion of variation (~50%) in plant assemblages was explained by the broader-scale spatial unit of experimental blocks, suggesting that broader-scale environmental factors have a greater impact on species composition in these communities than the very localized fine-scale impacts of CWM (~10%). Our study provides empirical evidence that species-specific responses to spatial heterogeneity align with spatial variation in community composition, consistent with the prerequisite necessary for the operation of spatial coexistence mechanisms. Fine-scale spatial heterogeneity contributes to spatial variation in overall species composition, though its effect is relatively smaller than that of broader-scale spatial heterogeneity. Together, our findings suggest that plant community composition is structured through the combined influence of fine- and broader-scale spatial processes.