Can we Constrain Carbon Assimilation and Allocation in a Multi-Species Hardwood Forest Using Water Flux Measurements?

Annual carbon budgets of terrestrial ecosystems and how climate perturbations alter them remain an active research area. A combination of measurements collected at multiple spatial and temporal scales is used in conjunction with models to quantify the relationship between water fluxes and C budgets. A multi-layer model for canopy CO2 uptake is employed in which the primary input is mean canopy stomatal conductance scaled via sap-flux of water vapor (gw) in a multi-species hardwood forest stand at the Duke Forest, NC, USA. The ecophysiological model relates stomatal conductance of CO2 (gCO2) to the ratio of internal (Ci) to external CO2 concentration (Ca) that is then used to calculate net assimilation (Anet) after correction for differences in diffusivities. Modeled assimilation rates agreed well with instantaneous leaf level measurements in the upper canopy, collected via porometry and monthly daytime carbon fluxes measured via eddy-flux augmented with daytime soil and wood respiration. Additionally annual biomass production augmented with construction and maintenance respiration agreed well with annual carbon uptake. The combination of sapflux measurements and the model provide reliable constrains on CO2 budgets in terrestrial ecosystems and showed lower carbon uptake in hardwood forest of the southeast than previously published.