Spatial variability in the vegetation structure and composition of an East African rain forest

We conducted a study of spatial variation in tree community structure and composition in the Kakamega Forest of western Kenya. We compared the tree communities at two sites, Buyangu and Isecheno, separated by approximately 11 km of contiguous forest. All trees ‡15 cm d.b.h. were censused along transects covering 4.95 ha at Buyangu and 4.15 ha at Isecheno. The structure of the forest at the two sites was similar as mean d.b.h. was comparable and stem size class distribution did not differ significantly. However, species richness and stem density were both much higher at Buyangu. The disparity in stem density may be because of the greater abundance at Isecheno of a semi-woody undergrowth species, Brillantaisia nitens Lindau, believed to inhibit the establishment of tree seedlings. Floristic composition varied strikingly between sites, with 52% of the species occurring only at one site. Densities of those species present at both sites often differed markedly between sites. Potential sources of these intersite differences in floristic composition include small disparities in rainfall, soil composition, elevation, and temperature as well as past differences in anthropogenic disturbance and in large mammal distribution and abundance. Floristic differences between Buyangu and Isecheno appear to be at least partly responsible for the substantial dietary differences between redtail monkeys (Cercopithecus ascanius Audebert) at these sites.