Coexistence and divergence of tropical dry forests and savannas in southern Mexico

Aim: The purpose of the study was to assess the degree of floristic differentiation between tropical dry forest (TDF) and savanna occurring in a single landscape. This comparison provides information on the responses of vegetation to the prevailing environmental conditions, while it also allows us to make inferences about large-scale events and processes, both biogeographical and evolutionary. Our approach included three levels of analysis: (1) taxonomic, (2) morphological and (3) vegetational. Location: The seasonal dry tropical landscape in the Nizanda region, Oaxaca State, southern Mexico. The landscape comprises a complex vegetation mosaic in which tropical dry forest and savannas are the most conspicuous components. Methods: Comparisons between TDF and savanna were based on inventories for these communities produced after 8 years of botanical survey. At the taxonomic level, the relative representation of taxa of different hierarchical levels in each community was examined. Morphological analyses required the classification of species on each of three criteria: (1) growth form, (2) life form and (3) growth habit. Vegetation level analysis was based on the frequencies of taxa in one hundred 100-m(2) composition plots with which matrices of binary data were constructed for species, genera and families. These were subjected to classification analysis with Ward's method and using Euclidean distances as the dissimilarity algorithm. Results: The combined flora for both communities comprised 600 species, 375 genera and 94 familie