Habitat associations of trees and seedlings in a Bornean rain forest

1 In species-rich mixed dipterocarp rain forest in Indonesian Borneo, we evaluated evidence for tree species associations with physical habitat variables (physiography for adults and seedlings, and light for seedlings). A total of 325 species were included in the analysis. 2 A stratified random sample of 28 tree and seedling plots (0.16 ha and 36 m(2), respectively) was taken from a 150-ha study area on homogeneous parent rock (granite) between 110 and 270 m a.s.l. 3 In a principal component analysis (PCA), based on an abundance index of all tree species, plots segregated into three groups. These corresponded closely with the three physiographically defined habitat types assigned in the field: (i) plateaux with a deep humus layer, (ii) sharp ridges and upper slopes, and (iii) gullies and lower slopes near permanent streams. Segregation of plots by habitat was weaker when seedling (rather than tree) species abundance was used in the PCA. 4 In single species analyses, using Monte Carlo randomization tests, we found significant associations with the three physiographic habitat types in 17 out of 49 abundant species for trees, and in 5 out of 22 abundant species for seedlings. In PCA and single species analyses, weaker associations with habitat for seedlings than for trees suggest higher mortality of seedlings in 'suboptimal' habitats. 5 Seedlings of 8 out of 45 abundant species were also significantly and positively associated with high light availability (measured in 2 x 2 m subplots, using hemispherical photographs). 6 Combining light and physiographic habitat associations, 20 out of the 45 abundant species were associated with at least one habitat factor as either adults or seedlings. 7 Thus, the distributions and abundances of many species are influenced by local heterogeneity in physical habitat variables. However, about half of the abundant species (25 of 45) showed no significant habitat association as adults or seedlings, and in no case did the relative abundance of any species exceed 5% in its 'preferred' habitat.