Recent studies of the phylogenetic structure of forest communities indicate that tree communities are often phylogenetically clustered (i.e., species are more closely related than expected by chance) at the local scale.  This is commonly explained as the result of environmental filtering of phylogenetically conserved traits leading to closely related tree species exhibiting similar habitat preferences.  To test this assumption, we examined how beta diversity and phylogenetic beta diversity are related to soil nutrient concentrations in eight 24-50 ha tropical forest plots located around the world.  Beta diversity measures how community composition changes across space, while phylogenetic beta diversity measures how the phylogenetic relatedness of the tree community changes across space.  We performed partial Mantel analyses of the effect of soil chemistry on both types of beta diversity, while controlling for spatial autocorrelation in the tree community, for small (20x20m) subplots within each study site.  We also divided the tree community into juvenile (1-10cm dbh) and adult (>10cm dbh) tree communities and repeated the above analysis on each age group separately.  

Results/Conclusions

For the entire tree community, beta diversity was found to be significantly related to soil chemistry in seven plots, and phylogenetic beta diversity was significantly related to soil chemistry in six plots.  The results indicate that sites with more similar soil characteristics have more similar tree communities, both in terms of the species composition and the overall phylogenetic relatedness of the tree communities.  In the comparison of this effect between juvenile and adult trees, we found the effect of soil on community composition is strongest among small trees, and the results are consistent across both beta diversity measures and most plots.  This indicates that the effect of environmental filtering due to soil resources is more prevalent at the juvenile stage than at the adult stage.  Taken together, the results indicate that soil resources act as an important environmental filter that shapes both the species distributions and the phylogenetic structure of these diverse tree communities.