Gross primary productivity in Duke Forest: Modeling synthesis of CO2 experiment and eddy-flux data

This study was designed to estimate gross primary productivity (GPP) in the Duke Forest at both ambient and elevated CO2 (ambient + 200 muL/L) concent rations using a physiologically based canopy model. The model stratified th e canopy of loblolly pine (Pinus taeda L.) forest into six layers and estim ated photosynthesis in each layer according to the Farquhar submodel couple d with the Ball-Berry stomatal conductance submodel. The model was paramete rized with a suite of physiological measurements, including leaf area index (LAI), leaf nitrogen (N) concentration, photosynthesis-N relationships, an d stomatal conductance. The model was Validated against measured leaf photo synthesis and canopy carbon (C) fluxes estimated from eddy-covariance measu rements (ECM). Application of this model to simulate canopy C fixation from 28 August 1996, the onset of CO2 fumigation, to 31 December 1998 suggested that elevation of atmospheric [CO2] to ambient + 200 muL/L resulted in inc rease of canopy C fixation by 35% in 1996, 39% in 1997, and 43% in 1998. Th e modeled GPP and its response to elevated [CO2] were sensitive to paramete r values of quantum yield of electron transport, leaf area index, and the v ertical distribution of LAI within the canopy. Thus, further investigation on those parameters will help improve the precision of estimated ecosystem- scale C fluxes. Furthermore, comparison between the modeled and ECM-estimat ed canopy C fluxes suggested that soil moisture, in addition to air vapor p ressure, controlled canopy photosynthesis during the drought period.