Species pool functional diversity and environmental heterogeneity jointly influence beta-diversity in temperate forests
A key goal in ecology is to disentangle how multiple processes at different scales influence patterns of biodiversity. Variation in biodiversity can arise from local-scale niche-based processes such as competition and environmental filtering as well as from regional-scale processes such as dispersal and speciation. However, the extent to which the assembly of communities is driven by local-scale processes, regional-scale processes, or the interplay between them remains unresolved. At local scales, assembly mechanisms are often inferred from patterns of functional-trait diversity. At regional-scales, in contrast, few studies have explicitly integrated functional traits into studies of community assembly. Here, we examined patterns of beta-diversity (spatial variation in species or functional composition) across three sites that differed in regional species-pool functional diversity to partition the relative importance of local- and regional-scale processes on the assembly of woody plant communities. Within the Smithsonian ForestGeo network, we compared taxonomic and functional beta-diversity across three biogeographic regions predicted to vary in species-pool functional diversity: western coniferous forest; central deciduous forest; and eastern deciduous forest. We used a null-modeling approach to examine how environmental heterogeneity (soils & topography) and species-pool functional diversity (leaf, stem and seed traits) may jointly influence patterns of beta-diversity.
The three forests varied in environmental heterogeneity, functional beta-diversity, and species-pool functional diversity. However, environmental heterogeneity alone could not account for observed differences in functional beta-diversity among forests. Functional beta-diversity was highest in the western coniferous forest with the lowest environmental heterogeneity, and lowest in the central deciduous forest with the highest environmental heterogeneity. Moreover, species-pool functional diversity was also highest in the western coniferous forest and lowest in the central-deciduous forest, suggesting an important influence species-pool functional diversity on patterns of beta-diversity. Indeed, after accounting for differences in species-pool functional diversity with a null model, we found that patterns of beta-diversity are more similar among forests than would be predicted if beta-diversity was solely due to local environmental variation. Our results suggest that greater trait diversity at the regional scale (higher species-pool functional diversity) may increase the potential for species to partition local-scale environmental variation, leading to greater spatial variation in biodiversity (beta-diversity) within a site. Our study provides new insights into how local and regional processes jointly influence patterns of forest biodiversity and ecosystem services that depend on trait variation among co-occurring species.