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 families; between them they shared 31, 40 and 34 taxa, respectively. The corresponding Sorensen similarity values were 10%, 21% and 72%, respectively. Ranking genera and families according to their species richness displayed large differences between savanna and TDF. Large differences between these communities were observed for Acanthaceae, Cactaceae, Euphorbiaceae and Mimosaceae, whereas Fabaceae and Asteraceae had similar high ranks according to the species richness in the two systems. The growth form spectrum diverged between the two communities, with TDF having more trees, shrubs and climbers. Savanna was characterised by forbs and graminoid herbs. Growth habit spectra revealed a clear dominance of herbaceous and suffruticose plants in savanna, and of woody elements and epiphytes in TDF. Regarding Raunkiaer's life forms, savanna had relatively more hemicryptophytes, and TDF more phanerophytes. Classification analyses showed that savanna and TDF forest samples kept their identities, regardless of taxonomic level (species, genera and families) at which the analyses were performed. Main conclusions: The TDF and savanna of Nizanda represent two floristic systems with a large degree of differentiation at all taxonomic levels and patterns of morphological attributes. This suggest that the two floristic sets have evolved independently for extended periods of time, despite their close proximity. One important implication of this floristic differentiation is the large joint contribution made by these communities to the regional flora.