Patterns of tree diversity in the tropical dry forests of Pacific coast Mexico

The tropical dry forest (TDF) is characterized by both extreme seasonality of rainfall and rich biological diversity. Nowhere is that diversity richer than along the Pacific coast of Mexico. In this region, tree species in particular display tremendous diversity, representing a substantial fraction of the angiosperm evolutionary tree. Of particular interest to plant ecologists and phytogeographers is how this diversity comes together in individual communities and how that changes across an elevation gradient. In this dissertation, I look at the tree diversity of the TDF at two different sites---Oaxaca and Jalisco---from three angles. First, I took an ecological perspective, looking at how movement along an elevation gradient affects community composition, species richness, associations, and turnover (beta diversity) from one site to the next. I found that although similar in terms of shared species, the Oaxaca site was considerably richer at all taxon levels than the Jalisco site, and that diversity increased with elevation---a trend not seen in Jalisco. Second, I examined diversity from a biogeographic perspective, looking at how geographic range size and center of range relate to abundance and presence in different communities. The main findings were that local abundance was not positively correlated with range size. In fact many locally abundant species had small ranges. Also, most species found in the sites occupied the majority of the dry forest habitat in Mesoamerica. Those species that made it into South America or were from there occupied a majority of the TDF in that continent. Third, I took a phylogenetic perspective, looking at how the relatedness of species in a given community changed with elevation and climate variables. The remarkable finding here was not that there was a strong phylogenetic signal across the elevation gradient, but that community relatedness was negatively correlated with elevation. This result forced a rethinking of the factors that drive phylogenetic clustering and dispersion. I hypothesized that historical climate fluctuations during the Quatemary, rather than current ecological conditions, are responsible for the patterns observed.