GROSS PRIMARY PRODUCTIVITY IN DUKE FOREST: MODELING SYNTHESIS OF CO2EXPERIMENT 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 μL/L) concentrations using a physiologically based canopy model. The model stratified the canopy of loblolly pine (Pinus taeda L.) forest into six layers and estimated photosynthesis in each layer according to the Farquhar submodel coupled with the Ball-Berry stomatal conductance submodel. The model was parameterized with a suite of physiological measurements, including leaf area index (LAI), leaf nitrogen (N) concentration, photosynthesis–N relationships, and stomatal conductance. The model was validated against measured leaf photosynthesis and canopy carbon (C) fluxes estimated from eddy-covariance measurements (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 μL/L resulted in increase of canopy C fixation by 35% in 1996, 39% in 1997, and 43% in 1998. The modeled GPP and its response to elevated [CO2] were sensitive to parameter values of quantum yield of electron transport, leaf area index, and the vertical 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-estimated canopy C fluxes suggested that soil moisture, in addition to air vapor pressure, controlled canopy photosynthesis during the drought period.