Drivers of tree species distribution across a tropical rainfall gradient

Understanding the factors shaping species distribution patterns along tropical rainfall gradients is necessary to predict the consequences of climate change for tropical tree communities. Direct effects of water availability exclude wet forest species from dry forests, but the exclusion of dry forest species from wet forests remains unexplained. We tested the hypothesis that high light and nutrient requirements exclude dry forest species from dark, infertile, wet forests. We transplanted seedlings of 26 woody species to six sites along a pronounced regional rainfall gradient across the Isthmus of Panama. We examined the effects of soil moisture, phosphorus and light availability, and species’ drought resistance on seedling performance, and linked the results directly to known species distribution patterns. Surprisingly, seedlings of wet forest species did not exhibit a home advantage: All species survived better under moister conditions, and the effects of phosphorus availability and light on seedling performance did not differ among species from dry or wet forests. Instead, dry forest species had intrinsically slower growth rates than wet forest species, which may lead to their exclusion from wet forests at later life history stages. High phosphorus exacerbated susceptibility to drought, although the mechanism remains unknown. Overall, our results demonstrate that seedling performance across tropical rainfall gradients is determined primarily by variation in soil water availability across space and time, while variation in nutrient and light availability plays a lesser role. Future changes in rainfall patterns will therefore have direct and pervasive consequences for forest composition and ecosystem function.