PNNL

Abstract

The carbon and water cycles are intimately linked in forested ecosystems. In tropical forests, their interactions may be important not only for controlling energy and gas exchanges directly but also for determining the competition among species, successional dynamics, forest structure and composition. However, such interplays have received limited attention in Earth system model development, as plant physiological traits mediate carbon dynamics while soil hydrological properties mainly regulate soil water dynamics. Here we use a vegetation demographic model, the Functionally Assembled Terrestrial Ecosystem Simulator (FATES) implemented in the Energy Exascale Earth System Model (E3SM) Land Model (ELM), ELM-FATES, to examine the sensitivity of tropical forest dynamics to plant traits and hydrological parameters concurrently. A large ensemble of simulations with perturbed parameters are conducted to explore how hydrological processes affect forest growth and species competition by validating against various observations of carbon, energy, and water fluxes from the Barro Colorado Island, Panama. Simulations reveal important interactions between belowground and aboveground processes. A greater fraction of deeper roots can lead to a more realistic capture of dry-season soil moisture and plant gas exchange, by allowing trees to extract water from deep soil layers. Soil hydrological parameters, in particular the scaling exponent of the soil retention curve (Bsw), play a crucial role in controlling forest diversity, with higher Bsw values (> 7) favoring late successional species in competition, and Bsw values between 1 and 7 promoting the coexistence of early and late successional plants. Improving carbon-hydrology interactions can resolve a systematic bias of FATES in simulating sensible/latent heat partitioning with repercussion on water budget and plant coexistence. As hydrological parameters are as important as vegetation parameters in predicting tropical forest dynamics and composition, more efforts are needed to improve parameterizations of soil functions and belowground processes and their interactions with carbon dynamics.