Empirical and optimal stomatal controls on leaf and ecosystem level CO2 and H2O exchange rates

Linkage between the leaf-level stomatal conductance (gₛ) response to environmental stimuli and canopy-level mass exchange processes remains an important research problem to be confronted. How various formulations of gₛ influence canopy-scale mean scalar concentration and flux profiles of CO₂ and H₂O within the canopy and how to derive ‘effective’ properties of a ‘big-leaf’ that represents the eco-system mass exchange rates starting from leaf-level parameters were explored. Four widely used formulations for leaf-level gₛ were combined with a leaf-level photosynthetic demand function, a layer-resolving light attenuation model, and a turbulent closure scheme for scalar fluxes within the canopy air space. The four gₛ models were the widely used semi-empirical Ball-Berry approach, and its modification, and two solutions to the stomatal optimization theory for autonomous leaves. One of the two solutions to the optimization theory is based on a linearized CO₂-demand function while the other does not invoke such simplification. The four stomatal control models were then parameterized against the same shoot-scale gas exchange data collected in a Scots pine forest located at the SMEAR II-station in Hyytiälä, Southern Finland. The predicted CO₂ (Fc) and H₂O fluxes (Fₑ) and mean concentration profiles were compared against multi-level eddy-covariance measurements and mean scalar concentration data within and above the canopy. It was shown that Fc comparisons agreed to within 10% and Fₑ comparisons to within 25%. The optimality approach derived from a linearized photosynthetic demand function predicted the largest CO₂ uptake and transpiration rates when compared to eddy-covariance measurements and the other three models. Moreover, within each gₛ model, the CO₂ fluxes were insensitive to gₛ model parameter variability whereas the transpiration rate estimates were notably more affected. Vertical integration of the layer-averaged results as derived from each gₛ model was carried out. The sensitivities of the up-scaled bulk canopy conductances were compared against the eddy-covariance derived canopy conductance counterpart. It was shown that canopy level gₛ appear more sensitive to vapor-pressure deficit than shoot-level gₛ.

Duke Scholars

Author Gabriel G. Katul Civil and Environmental Engineering