Patterns of Tree Species Richness in Rain Forests of the Middle Caqueta Area, Colombia, NW Amazonia

Abstract

Tree species, genus, and family richness (DBH 2 10 cm) is recorded in 95 plots of 0.1 ha, situated in most physiographic units in the middle Caqueta area, east of Araracuara, in the Amazon lowlands of Colombia. A positive correlation is present between forest canopy height and tree species density. Highest tree taxon density is recorded in upland (non-flooded) plots with well-drained soils. Lowest tree taxon density is found in poorly drained swamps, while plots on podzolized (white sand) soils show intermediate levels. In well-drained flood plains tree taxon density is lower in comparison with well-drained uplands, but this difference becomes smaller when stem densities are taken into account. Patterns of cumulative numbers of tree taxa in series of plots combined are similar. The results suggest that general patterns of tree taxon richness are mostly related to habitat hostility and environmental stress factors. Fluvial dynamics, between plot soil heterogeneity and beta diversity, play only a subordinate role in the maintenance of the high tree species diversity in the well-drained uplands. Here, tree species density in proportion to stem density is slightly higher in forests on the less nutrient poor, more dayey and reddish soils with thinner humus forms compared to forests on very poor, loamy, and yellowish soils with thicker humus forms. These results may be explained in terms of habitat hostility (stress) or forest dynamics. Gap phase dynamics could be important in maintaining high levels of tree species alpha diversity in well-drained upland forests. Extinction of tree species related to cydic forest disappearances on a regional scale has probably been relatively unimportant.