The environment and space, not phylogeny, determine trait dispersion in a subtropical forest

A central aim of forest ecologists is to quantify the relative importance of different community assembly mechanisms in tropical and subtropical tree communities. Recent work in this field has focused on the importance of functional trait similarity and abiotic filtering. While important, none of this work has simultaneously: linked these trait dispersion patterns to the underlying abiotic environment, considered dispersal limitation and quantified the degree to which patterns of trait dispersion may be explained simply by shared ancestry. Here we use data from a subtropical Chinese forest to accomplish this goal. We first examine the trait dispersion (leaf area, specific leaf area, seed mass, wood density, maximum height and five traits together) on local scales by comparing the observed trait dispersion pattern to that expected from a null model. Then we use a variance partitioning approach to examine the degree to which spatial proximity, environmental similarity or the phylogenetic dispersion of the species determine the observed trait dispersion. The results show that, on local scales, trait dispersion is often non-randomly filtered. Further the widespread trait clustering observed is largely explained by the environment and space, while the phylogenetic dispersion of species in a sample explains relatively little. This result further underscores that inferring an assembly mechanism from a pattern of phylogenetic dispersion is tenuous. The work is important in that it is the first to partition the variation in tree trait diversity into its spatial, environmental and phylogenetic components and that it demonstrates that functional trait data often lack enough phylogenetic signal on local scales to confidently link patterns of trait and phylogenetic dispersion. Ultimately, the findings suggest a strong role for abiotic filtering and dispersal limitation during community assembly on local spatial scales and that shared evolutionary history plays a relatively small role.