Habitat associations of trees and seedlings in a Bornean rain forest

1 In species-rich mixed dipterocarp rain forest in Indonesian Borneo, we ev aluated evidence for tree species associations with physical habitat variab les (physiography for adults and seedlings, and light for seedlings). A tot al 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 paren t rock (granite) between 110 and 270 m a.s.l. 3 In a principal component analysis (PCA), based on an abundance index of a ll tree species, plots segregated into three groups. These corresponded clo sely with the three physiographically defined habitat types assigned in the field: (i) plateaux with a deep humus layer, (ii) sharp ridges and upper s lopes, and (iii) gullies and lower slopes near permanent streams. Segregati on 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 fou nd significant associations with the three physiographic habitat types in 1 7 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 wit h habitat for seedlings than for trees suggest higher mortality of seedling s in 'suboptimal' habitats. 5 Seedlings of 8 out of 45 abundant species were also significantly and pos itively associated with high light availability (measured in 2 x 2 m subplo ts, 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 t he abundant species (25 of 45) showed no significant habitat association as adults or seedlings, and in no case did the relative abundance of any spec ies exceed 5% in its 'preferred' habitat.