Modeling vegetation function is significant for global change biology as it provides a simple theoretical framework for analyzing and predicting climate impacts on biospheric carbon and hydrological cycles under environmental change. Simulation of the process of carbon uptake provides foundation for those vegetation function models. In this work, we explore a modified CO2 assimilation model that is based on the Canopy Conductance Constrained Carbon Assimilation model (4CA). Similar to its predecessor, the new 4CA model is a multi-layer model that accounts for light attenuation in the canopy. However, it uses a stomatal optimization approach to estimate canopy conductance, therefore reducing the number of parameters required to estimate whole crown carbon assimilation. The integrated optimized conductance is then constrained by measured sap-flow based canopy conductance, and carbon assimilation is calculated with a Farquhar-type model under CO2 and light co-limitations.
We aimed to understand the relationship between sap-flow based conductance and the computed optimized conductance, analyze the effect of marginal water use efficiency (λ) on carbon assimilation estimation, compare carbon assimilation and water use efficiency calculated by the new model with previous models, and elucidate the difference in water use efficiency (WUE) derived from various methods, including modeled canopy water use efficiency, isotopically derived WUE, and instantaneous leaf gas exchange measurement derived WUE.
The data used were collected at the Rockaway River State Wildlife Management Area (Jefferson Township, NJ, USA) during the summer of 2016, and included sap flow measurements on 8 trees of the dominant hickory species using the Granier method, meteorological data, and representative leaf gas exchange measurements.
Our preliminary results show that the optimal conductance presents similar behavior to the sap-flow based conductance during the study period, upon choosing the appropriate λ value. On the other hand, as the conductance was constrained to the sap-flow based conductance, ultimately, λ value has a minimal impact on net assimilation. Leaf gas exchange water use efficiencies ranged from 2.6 to 3.4 mmol mol-1 and canopy water use efficiencies ranged from 5.3 to 6.4 mmol mol-1 depending on weather condition, tree size and λ. Thus, canopies exhibit a higher water use efficiencies than individual trees.