Causes and ecosystem consequences of tree species turnover along soil nutrient gradients in lowland rain forest of Indonesian Borneo

Beta diversity (or species turnover) along environmental gradients is an important component of plant diversity in tropical rain forest, yet we know little about its causes or implications for ecosystem dynamics. This dissertation investigates the causes and ecosystem consequences of species turnover throughout a continuous lowland Bornean rain forest (∼340 ha) underlain by three distinct parent materials. I combine direct measurements of soil nutrient pools, species composition, and aboveground productivity to quantify landscape-level variation in community composition, ecosystem productivity and the efficiency of plant nutrient uptake and use. I test the hypothesis that high beta diversity throughout this landscape is a result of deterministic processes that favor species with contrasting plant growth strategies on rich versus poor soils.; Steep gradients of soil nutrients (especially P) were quantified throughout the study area and were associated with major shifts in the species composition of canopy trees in the Dipterocarpaceae, the dominant plant family. Species distributions varied significantly with soil P, and to a lesser degree Mg and Zn. Concomitant changes in aboveground net primary productivity were also observed along the nutrient gradient---productivity varied three-fold and increased significantly with extractable P and exchangeable Ca. Variation in growth of large trees (≥60 cm diameter) explained spatial patterns of productivity throughout the site and determined the overall relationship between soil P and aboveground productivity.; Although communities were less productive on P-poor soils, they were more efficient in P uptake and use than communities on P-rich soils. Lower productivity but higher P efficiency on P-poor soils appeared to be a reflection of intrinsic trait differences between dominant species in communities on different soil types. Pairwise phylogenetically independent contrasts showed that dominant dipterocarp species on P-poor soils had slower intrinsic growth rates but produced leaves indicative of higher nutrient use efficiency than species on P-rich soils.; Thus, beta diversity throughout this tropical landscape was high and appeared to be caused in part by ecological trade-offs associated with contrasting plant growth strategies. The spatial segregation of species with divergent traits had important consequences for landscape-level patterns of plant diversity and ecosystem processes throughout this Bornean rain forest.