Abiotic and biotic spatial variation has long been thought to mediate the outcome of competitive interactions and community dynamics, and plant ecologists are increasingly using functional traits to understand associations between plant performance and neighborhood interactions. However, we lack a clear understanding of how variation in plant functional traits is related to the species’ differences in their demographic response to heterogeneous environments. Therefore, a better understanding of the relationship between trait differences and variation in demographic parameters across space is required to link functional trait patterns to underlying coexistence processes. The spatial storage effect can drive coexistence in a patchy landscape by combining buffered population growth with species-specific responses to environmental variation. Using an annual plant community in coastal California, we experimentally estimated demographic parameters to quantify average fitness differences for 136 species pairs at 24 sites across an edaphic and temperature gradient. We then asked whether differences in leaf, root, seed, and whole-plant traits are predictive of landscape-level average fitness differences that drive competitive exclusion. Finally, we tested whether variation in the soil microbiota contributes to fitness differences in this system by conducting a growth-chamber experiment in which we grew eight species in field-collected soil that had been sterilized.
We found that approximately 20% of species pairs had negatively correlated demographic responses across the 24 sites in our experiment, with the demographic responses being germination and seed production in the absence of competition. This shows that an important prerequisite for the spatial storage effect is operating in our focal community at the landscape scale. Further, the degree to which species pairs responded to the environment in similar ways was correlated with differences in key functional traits, including phenology, plant stature, leaf dry matter content and leaf nitrogen. Our growth chamber experiment suggests that microbial activity in this landscape also has species-specific impacts on plant performance. This study highlights the importance of understanding the links between functional traits, biotic and abiotic environmental variation, and the demographic drivers of competitive interactions at the landscape scale.