A stomatal optimization approach improves the estimation of carbon assimilation from sap flow measurements

Abstract

Modeling the carbon uptake process by plants provides the foundation for global vegetation models. We modified the Canopy Conductance Constrained Carbon Assimilation (4C-A) model, a multi-level assimilation model that utilizes sap-flux measurements, based on the stomatal optimization theory. A novel method of calculating optimal stomatal conductance and carbon assimilation (Anet), which considers the co-limitation of Rubisco activity and RuBP regeneration, has now been incorporated into the modified 4C-A model. Results show that the calculated daily average Anet by the modified 4C-A model was lower than that from the original 4C-A and a big-leaf model but agreed well with that from leaf gas exchange measurement. When the marginal water use efficiency (λ) was high, the calculated daily average Anet became lower than that derived from lower λ. Differences in Anet between the modified and original model were larger when the canopy exhibited higher stomatal conductance or VPD was lower. Hence, a more accurate Anet was estimated by the modified model at dawn and dusk or during humid days when low VPD occurred. In addition, the modified 4C-A model performed better on a diurnal timescale for Anet and stomatal conductance estimation compared to the original one. This modified 4C-A model provides a more realistic estimation of Anet on half hourly to daily timescales, and, thus, can be applied to elucidate the coupling between carbon and water cycles and test the responses of carbon assimilation to environmental change.