Drought resistance traits predict tree species performance in a humid tropical landscape, but their importance shifts between managed cover types

In the humid tropics, the substitution of forest cover by open pastures generates hotter and dryer conditions limiting the establishment of native plants. In this context reforestation of pastures using fast-growing tree species is commonly used to improve the environmental conditions, however, contrary to expected this strategy may lead to drier soil conditions due to the elevated transpiration rates of large trees, overall generating landscape mosaics with high variation in light and water availability. Despite the high sensitivity of tree species from the humid tropics to drought, efforts to predict species performance in reforestation campaigns have been focused mostly on carbon gain traits, while ignoring the drought resistance traits. In this study we addressed the hypothesis that both carbon gain and drought resistance traits are good predictors of growth rate and survival of species under contrasting vegetation cover types in a managed landscape. To test this hypothesis, the growth and survival over 38 months, and 17 physiological and morphological leaf traits related to carbon economy and drought resistance were measured in ten tree species planted in pastures and tree plantations. Plantations showed higher soil water depletion than pastures and the relative growth rate and survival of species were higher in pastures. Carbon economy and drought resistance traits together predicted interspecific growth rate and survival; however, the importance of functional traits as predictors depended on the cover type. Carbon acquisition traits (Am and AFE) were the best predictors in pastures, while drought resistance traits (RWCtlp, LDMC and epsilon) had the lowest magnitude of interspecific variation, but were the best predictors in plantations. This suggests that, despite its restricted variation, drought tolerance traits may be of paramount importance in defining tree performance and long-term success of managed species in the humid tropics, particularly under fast-growing tree covertures. This study shows that soil water might act as a limiting factor on plant performance in managed landscapes in the humid tropics, and challenges the common assumption that using a low diversity of fast-growing and highly resource-demanding species is the best strategy to reforest abandoned pastures and improve the environmental conditions for other species.