Floristic differentiation in limestone outcrops of southern Mexico and central Brazil: a beta diversity approach

Abstract

We studied the spatial arrangement of fl oristic diversity in two systems of limestone outcrops, located in two distant Neotropical sites: the region of Nizanda (S Mexico) and the Paranã Valley (Central Brazil). We addressed the question whether their vegetation could display a similar zonation, and we explored the relative effects of distance and an environmental gradient on α-, β- and γ-diversities. The limestone outcrops at both sites are similar in size and in elevation, but strongly differ in between-outcrop distance by an order of magnitude. At each study site three individual limestone outcrops were selected; in each of them three plant communities along the edaphic gradient were distinguished (a xerophytic scrub and two tropical dry forests types, one of which had a more xeric character than the other), and sampled for structural variables and fl oristic composition in 100-m2 plots. At both study sites, structural variables responded similarly to the edaphic gradient. Species density was larger in Nizanda for both α- and γ-diversity, but the largest value of β-diversity was obtained in Paranã. The edaphic gradient produced larger mean β-diversity values than the simple distance effect, with the interaction of both factors resulting in an even larger β-diversity. Classifi cation analyses by site showed larger fl oristic similarities between the two xerophytic communities than those existing between them and the more mesic forests. The spatial arrangement of diversity showed that both α- and γ-diversity were smaller for the xerophytic communities. As hypothesized, the more extreme changes in community physiognomy were associated with larger β-diversity values.