Interspecific differences in determinants of plant species distribution and the relationships with functional traits

1. Environmental control and dispersal limitation are both essential processes in plant community assembly and species distribution. Although numerous studies in the past decade have examined their importance as determinants of community composition, remarkably little is known about interspecific differences in the importance of these two processes. 2. To quantify these interspecific differences, we compared the importance of environmental factors and space as correlates of species distribution among 24 understorey plant species in a Japanese cooltemperate forest by performing variation partitioning at the species level. Specifically, we hypothesized that the importance of environment and space differs among species, and these differences can be partly predicted from the functional traits and/or phylogenetic identity of each species. 3. The unique contributions of both environment and space were significant in the community-level analysis. However, at the species level, the relative and absolute sizes of the unique contributions of environment and space differed considerably among the 24 species. Environment and space were not necessarily significant variables explaining the distribution of many species. 4. No significant relationships were found between the unique contribution of environment and the four functional traits tested, that is, dispersal mode, seed mass, plant height and specific leaf area among the 24 species. In contrast, the unique contribution of space was significantly larger in species with no dispersal mechanisms than in animal-dispersed species. No significant phylogenetic signal was detected for the unique contribution of environment or space, suggesting that importance of environmental control and dispersal limitation as determinants of species distribution is evolutionarily labile. 5. Synthesis. Our results suggest that the relative and absolute importance of different processes of community assembly (i.e. environmental control and dispersal limitation) differs remarkably among species even within a single community. These interspecific differences may be explained in part by interspecific differences in dispersal mode.