I. Pedological and botanical variations across the forest-savanna transition on Maraca Island

Three major types of forest-savanna boundary occur on Maraca Island. The most widespread in the east of the island consists of a forest-hyperseasonal savanna transition often with a narrow band of drier, arboreal savanna dominated by Curatella americana and Byrsonima crassifolia fringing the forest. At these locations, the boundary coincides closely with the upper limit of the seasonal water table within the rooting zone. This boundary feature is emphasized wherever a slight toposequence concentrates the water saturation over a narrow vertical range in the profile. The relative stability in the horizon can be detected from well-marked mottling within the profile. A second type occurring throughout the island and easily identified on satellite images, is the 'vazante'. Vazantes are valley-like features containing a dense layer of herbs, herbaceous climbers and grasses and occupying seasonal stream lines through tall forest. The forest-marginal shrub boundary is extremely sharp and is coincident not only with the morphologically distinct seasonal stream lines but also with a very characteristic soil change from welldrained sandy soils in the forest to poorly drained clay-rich soils in the vazantes. A third group consists of isolated islands of low arboreal savanna. Although the basic determinant for savanna in this location seems to be oligotrophic, deep, coarse-textured soil; there may also be a periodic flooding influence. These savanna islands appear to have an irregular linear distribution which may represent a palaeofluvial course still occasionally prone to flooding.