Tropical forests store more carbon in aboveground biomass (AGB) than any other terrestrial ecosystem on the planet. This carbon is largely stored in trees belonging to a range of plant functional types (PFTs) that vary in their sensitivity to light and water resources. There is considerable uncertainty between climate models in their predictions of how precipitation patterns may change over the tropics by the end of this century. Furthermore, the implications of such changes on tropical forest AGB are unclear. Land surface models that include demographic and plant hydrodynamic processes, such as the Ecosystem Demography model (ED2-hydro), are promising tools for understanding how different precipitation scenarios may affect carbon storage across different PFTs. In this study, ED2-hydro was driven with local meteorological drivers reconstructed to represent several of the predicted, yet contrasting, precipitation scenarios, including less variation, longer dry seasons, El Nino related droughts, drier dry seasons, and drier wet seasons. The meteorological drivers preserve the inverse correlation between rainfall and radiation that occurs when precipitation patterns change. ED2-hydro allows for dynamic competition between four PFTs—early- versus late-successional groups subdivided into drought-tolerant versus -intolerant groups—to occur along two largely orthogonal resource gradients of water and light.
ED2-hydro predicted the aboveground biomass responses of the four simulated PFTs to be markedly different under the various precipitation scenarios. Therefore, the model predicts that future changes in the mean or variation in plant available soil water (PAW) will alter the functional diversity of seasonally wet tropical forests. Furthermore, variation in PAW is regulated by both quantity and variability in precipitation and can be buffered in ecosystems with greater rooting depth. The model also predicts that total ecosystem AGB will only be marginally altered by most future precipitation scenarios that are predicted by climate models. The only precipitation scenario that leads to a significant reduction in AGB by the end of this century is an intensification of the dry season, but not necessarily an increase in the dry season. Model predictions are consistent with the intermediate disturbance hypothesis where an intermediate frequency in droughts has a stabilizing effect on functional diversity; but, functional diversity is inhibited by more frequent and extreme droughts or less variable precipitation patterns.