Floristic and structural contrasts between natural savannas and anthropogenic pastures in a tropical dry landscape

The magnitude of the biological differentiation between natural savannas and pastures (anthropogenic grasslands)coexisting in a single landscape, in terms of their floristic composition and community structure, was studied in the Nizanda region, Isthmus of Tehuantepec, Oaxaca State, southernMexico. Vegetation samples of 15m2 each were taken at 20 savanna and 11 pasture sites. Cover- and species richness-based diversity and dominance indices were calculated. Geomorphological and edaphic characteristics were also compared. Savannas generally occurred in hill summits with very shallow and stony, discontinuous and acidic soils; contrastingly, pastures were located in piedmonts and floodplains, with more neutral and less stony soils. Savanna sites differed from pasture sites in species richness and Shannon diversity. They did not differ in Simpson dominance index, but the identity of the dominant species was different in each community: Trachypogon spicatus (L.f.)Kuntze in savanna and Megathyrsus maximus (Jacq.)B.K.Simon et S.W.L.Jacobs in pasture. A low species-level similarity (Sørensen Index)was observed between them (8.2)