Improved representation of stomatal conductance in Functionally Assembled Terrestrial Ecosystem Simulator (FATES) model

Abstract:

Tropical forests play a key role in modulating global climate through the exchanges of water, carbon, and energy between the land and atmosphere. Regulation of these fluxes occurs principally through stomata. Therefore, accurate model representation of controls on stomatal aperture is of fundamental importance to simulating land-atmosphere interactions. The Functionally Assembled Terrestrial Ecosystem Simulator (FATES) is a state-of-the-art dynamic vegetation demography model that has the ability to simulate both detailed plant demographics and ecophysiological dynamics to study how forest succession, turnover, and change impacts ecosystem-scale functions. We implemented the “Unified Stomatal Optimization (USO) model”, that simulates the relationship between photosynthesis (A) and stomatal conductance (g_s) as an alternative to the default “Ball-Berry” method. In order to mechanistically understand how the implementation of the USO model might affect FATES model outputs, we conducted a model sensitivity analysis to explore the responses of g_s to climate forcing variables including atmospheric CO₂ concentration, air temperature, radiation, and humidity. We found that modelled g_s varied between the Ball-Berry and USO formulations especially under very moist conditions. However, during the sensitivity runs these differences did not result in a significant change in gross primary productivity or evapotranspiration. We then conducted simulations using observed forcing variables from a wet evergreen forest located in the San Lorenzo Protected Area, Panama (PA-SLZ). We compared our PA-SLZ simulations to field measurements over the same period and found that the USO model formulation was better at capturing the magnitude and diurnal change of measured g_s.