Patterns of phylogenetic dispersion in tropical dry forest trees and implications for community assembly

Background/Question/Methods

The tropical dry forests of Mexico exhibit both high floristic diversity and strong seasonality, inviting an analysis of the relative roles of biotic and abiotic factors in community assembly. We sampled tree diversity in plots across an elevation gradient at two widely-separated sites on the Pacific slope and compared the phylogenetic diversity in each sample with that of the site species pool. Because rainfall correlates positively with elevation, we hypothesized that there would be a phylogenetic signal showing community composition change across this gradient. For each site we used the species list to construct a phylogenetic supertree based on published relationships and the Angiosperm Phylogeny Group’s backbone tree. We then made pruned phylogenetic trees composed of the species found in each transect as sample communities. All trees were agnostic about branch length, using simple unity between nodes. We used Phylocom software to analyze the trends in the net relatedness index (NRI) and phylogenetic diversity of sample communities with respect to the supertree across the elevation gradient.
Results/Conclusions

Phylogenetic dispersion was negatively correlated with elevation at both the genus and species levels for both sites. In a spatial analysis using ~1 km climatic data, we confirmed the strong correlation between annual precipitation and NRI. Phylogenetic diversity, but not species richness, was also inversely related to elevation. These results contradicted expectations that higher temperatures, less rainfall and greater seasonality at low elevations would lead to phylogenetic clustering relative to upland sites. We hypothesize that climate fluctuations in the Quaternary made low elevation sites close to the ocean milder and more stable over intermediate time scales that would favor biotic regulation of community assembly relative to higher elevation inland sites that would be mediated by abiotic climate-related factors.