The nature of savanna heterogeneity in the Orinoco Basin

Detrended Canonical Correspondence Analysis (DCCA) was used to describe the relationship between savanna environments and vegetation community composi tion in the Orinoco Basin. TWINSPAN derived three major types of savanna ve getation, each with its own Aoristic and physiognomic features. The first g roup reflected a plant association with species Trachypogon occurring in mo derately infertile soils with low water availability and high bulk density. This group is distributed mostly on hilltops and dissected plains of the n orthern Orinoco savannas. The second group has the highest species diversit y in habitats occurring in the northern Orinoco with extremely infertile so ils. The third group, on the other hand, has a considerable number of speci es belonging to the leguminosae and Cyperaceae families and linked to the h abitats of the eastern Colombian savannas with the highest soil water avail ability, a shorter dry season and higher precipitation. The regional analysis of the Orinoco savannas suggests edaphic controls to be important. Changes in the underlying geology have affected topography an d soil formation, which results in variation in water and nutrient status. The moisture regime and hydrological features are acting as the major regio nal determinants, while nutrient levels and distinctive surface soil proper ties provide the subregional determinants. The local boundary, at any given site, is strongly determined by nutrient shortage and acidity. Also, bioge ographic and floristic considerations were taken into account to explain di fferences in species composition.