TREE SPECIES COMPOSITION AND RAIN FOREST-ENVIRONMENT RELATIONSHIPS IN THE MIDDLE CAQUETA AREA, COLOMBIA, NW AMAZONIA

As part of an integrated forest vegetation and soil survey, tree species composition (DBH greater than or equal to 10 cm) was recorded in 95 plots of 0.1 ha, distributed over the principal physiographic units in the middle Caqueta area, Colombian Amazonia. A total of 1077 tree species was found, classified into 271 genera and 60 families. Leguminosae and Sapotaceae show high familial importance values in all physiographic units. Lauraceae, Chrysobalanaceae, Moraceae, and Lecythidaceae are more important on well drained (flood plain or upland) soils, while Palmae, Guttiferae, Bombacaceae, and Apocynaceae are more important in swamps and on podzolised ('white sand') soils. Plots on well drained soils show a lower degree of dominance than plots in swamps or on podzolised ('white sand') soils. The composition of the most dominant species in the plots changes continuously. Most species (59%) are only recorded in one plot. Individual plot pairs generally show a low overlap of about 2-5 tree species, resulting in Jaccard coefficients below 20%. Complementary to a previous forest classification based on TWINSPAN analyses, detrended and canonical correspondence analyses were carried out, using CANOCO 3.1. Despite of a low amount of tree species variance explained (only 6.2% by the first two canonical axes), meaningful patterns of tree species composition were recognised. These are most strongly related to drainage, flooding, humus forms, and soil nutrient status. Forest types are well separated in the CCA ordination diagram. The most frequently found tree species are listed according to their preference with respect to drainage, flooding, and soil nutrient status. Tree species composition in the well drained upland forests was analysed separately. In view of the model explaining high NW Amazonian tree species diversity on the basis of dense community packing and high beta diversity along soil gradients, the canonical analysis here focused on the effect of soils. By means of partial canonical ordination it was found that patterns of tree species composition depended significantly on soil properties, even though the edaphic component explains only a small fraction of the tree species variance. The results show that the well drained uplands of the middle Caqueta area are covered by a complex of two intergrading tree species assemblages. The first assemblage (community of Goupia glabra-Clathrotropis macrocarpa) is associated to somewhat less poor, clayey soils developed in Andean origin deposits or Tertiary sediments from the Pebas formation. The second assemblage (community of Swartzia schomburgkii-Clathrotropis macrocarpa) shows affinities to very poor, loamy soils developed in parent materials derived from the Guiana shield. This simple dichotomous pattern of geology, soils, and forest types is incompatible with concepts of high soil heterogeneity and associated beta diversity controlling tree species diversity in well drained uplands of NW Amazonia. The gradient length of tree species in the detrended correspondence analysis was low (3.7 SD), also suggesting a low beta diversity.