The Science

Stomata regulate carbon dioxide (CO₂) uptake by photosynthesis and water loss through transpiration. Accurate model representation of this process, called stomatal conductance, is therefore key for modeling CO₂ and water fluxes. The approaches used to represent stomatal conductance in models vary. Current understanding of the drivers of the variation in a key parameter in those models—the slope parameter, which is a measure of plant water-use efficiency—is still limited, particularly in the tropics. Scientists from Brookhaven National Laboratory and the Next-Generation Ecosystem Experiments (NGEE)–Tropics team evaluated the ability of current model formulations to predict observed stomatal conductance, including the inclusion of leaf water potential, and investigated the sources of variation in the slope parameter. They found that inclusion of leaf water potential did not improve model predictions and that model formulations that included vapor pressure deficit performed better than those that relied on relative humidity.

The Impact

Although the value of stomatal slope can have a large impact on simulated carbon and water fluxes, the understanding of what drives the variation in slope parameter is still limited. This study presents a novel integration of rare measurements of gas exchange from the upper canopy of a tropical forest in Panama and a suite of plant traits with analysis that advances the understanding of dominant drivers of stomatal slope variability and identifies a practical, trait-based approach to improve modeling of carbon and water exchange in tropical forests.

Summary

Stomatal slope is inferred from an example stomatal conductance model. For a given CO₂ assimilation rate, atmospheric CO₂ concentration, and leaf-to-air vapor pressure deficit (collectively, the x-axis), a higher slope means that plants maintain a higher stomatal conductance (y-axis) for a given photosynthetic rate. As such, the slope parameter is an indicator of plant water use efficiency, and a greater slope equates to a lower water use efficiency. The team performed diurnal gas exchange measurements (resulting in background scatterplots) for two example species (Ventilago ferruginea and Terminalia amazonia).

Principal Investigator

Alistair Rogers
Brookhaven National Laboratory
arogers@bnl.gov

Program Manager

Daniel Stover
U.S. Department of Energy, Biological and Environmental Research (SC-33)
Environmental System Science
daniel.stover@science.doe.gov

Funding

This work was funded by the Next-Generation Ecosystem Experiments (NGEE)–Tropics project in the Terrestrial Ecosystem Science (TES) program of the Office of Biological and Environmental Research (BER), within the U.S. Department of Energy (DOE) Office of Science.

References