Background/Question/Methods A classic hypothesis to explain diversity in tropical forests is that species coexist by niche differentiation, trading-off performance in different habitats. Most tree species in a 52-ha forest dynamics plot in Borneo have distributions correlated with soil habitats, but functional mechanisms explaining these distributions are unknown. Previous analyses indicate that species are sorted among soils as trees age, perhaps mediated by variation among soils in the severity of an interspecific trade-off between growth and survival rates. If so, then ecophysiological adaptations for rapid growth may incur relatively higher survival costs on resource-poor compared to resource-rich soil. To test this hypothesis, we measured in situ variation in leaf functional traits related to resource-use efficiency and herbivore defense on saplings of 31 soil specialist and generalist species in congeneric pairs or triplets.

Results/Conclusions Overall, both specialists of sandy loam and populations of generalists growing on sandy loam (the soil lowest in mineral nutrients and water content) had thicker, tougher, longer-lived leaves with lower nutrient concentrations, higher C:N ratios, and greater water-use efficiencies relative to on the richer clay soil. Suites of traits covaried, suggesting coordinated ecological strategies, but not all trait variation was in the predicted direction. Hence, species with fast growth rates may have higher mortality rates o sandy loam soil for different reasons, suggesting that to predict distribution patterns, traits must be considered from a whole-plant perspective in relation to population growth rnates. By assessing natural variation in function with respect to performance, this research links ecophysiology with its consequences for forest community structure.