Grasses of the Precambrian Shield region in eastern lowland Bolivia. I. Habitat preferences

The habitat distribution and relative abundance of 113 species of Gramineae were documented by releves in 82 stands near Conception, Santa Cruz, Bolivia. A factor analysis was used to compare the floristic similarity of stands situated in semideciduous forest, cerradao, cerrado, campo rupestre, valley-side campo, pantanal complex or on granite outcrops. Individual grass species usually had a preferred habitat and occurred with decreasing abundance in stands judged to be transitional by physiognomic, edaphic, and floristic criteria. Stands situated in cerrado vegetation were most similar to one another in grass species composition. Cerradao was transitional to semideciduous forest and cerrado but certain grasses were characteristic of this vegetation type. The floristic composition of the single campo rupestre locality was somewhat similar to cerrado; however, several of the more abundant grass species of this vegetation type did not occur in any nearby cerrado stand. Granite outcrops had a distinct grass flora and showed little similarity to other vegetation types. In savanna wetland communities, grass species distribution was influenced by water regime. Differences between stands on valley-side campo corresponded to topographic position on a gradient of increasing water surplus. Seasonally humid/dry stands on valley-side campo and pantanal complexes had a high degree of similarity. Stands lower on the catenary sequence of pantanal complexes and valley-side campos were increasingly dissimilar, a result of the different edaphic conditions of the seasonally flooded soils of pantanal complexes when compared with the permanently saturated (but never flooded) soils of valley-side campos. Pantanal complexes had the richest grass flora of all vegetation types because their microtopographic variability creates numerous micro-habitats with distinct water regimes, each supporting different grass species.