| Abstract
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This study examined processes that may maintain high tree species richness in mixed dipterocarp forest at Gunung Palung (West Kalimantan, Indonesia). I sampled the forest with 28 plots (0.16 ha for trees, 36 m2 for seedlings) distributed over a 150 ha study area, and recorded 325 species of tree (usd\geusd10 cm DBH) and 441 species of seedlings (<1 cm DBH). The most abundant families were the Dipterocarpaceae, Euphorbiaceae, Clusiaceae, Myrtaceae and Myristicaceae. I focused on two major phenomena relevant to hypotheses of the maintenance of species diversity: (i) density dependence, and (ii) species association with habitat heterogeneity. I first assessed evidence for the dependence of seedling survival on the abundance of conspecific trees and seedlings over both wide (150 ha) and local (at 0.16 ha and 1 m scales. At the 150 ha scale, I identified a community-level compensatory trend in seedling survival (i.e., more abundant species had higher seedling mortality), suggesting that community-level density dependence, or frequency dependence, is contributing to species coexistence. On a local scale, seedling survival was inversely related to conspecific seedling density (in 1 m and tree basal area (in 0.16 ha), for 5 out of 15 abundant species, and for all species combined. I also examined species associations with habitat for trees and seedlings in the 28 plots. I identified 3 physiographic habitat types that influenced species composition: (i) plateaus with deep humus, (ii) ridges and upper slopes, and (iii) gullies and lower slopes. I found significant associations of trees with these 3 physiographic habitat types in 24 out of 48 abundant species. Seedlings were less strongly associated with particular habitats than adults, suggesting higher mortality of seedlings in 'sub-optimal' habitats. Seedlings of 8 out of 43 abundant species were also significantly and positively associated with high light availability Overall, the findings constitute strong evidence that density dependent processes contribute to coexistence in this diverse rain forest. Because species were found to respond differently to habitat heterogeneity, it is also possible that habitat partitioning occurs and promotes diversity.
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