Carbon balance of an oak/pine stand in the New Jersey Pine Barrens

We assessed the growing-season carbon balance of an oak / pine forest stand in the New Jersey Pine Barrens using the Canopy Conductance Constrained Carbon Assimilation model (4CA Model) in conjunction with eddy-covariance, sap flow and leaf-level photosynthetic gas exchange measurements at several canopy levels at the site. Despite poor sandy soil conditions, oak leaf nitrogen averaged 1.9 % throughout the 2006 season. This was expressed through by high mean stand-level carbon assimilation rates of 4 g C m^-2 day^-1 and with a maximum of 8 g C m^-2 day^-1 as modeled with 4CA.  4CA results were consistent with eddy-covariance net ecosystem exchange (NEE) CO₂ flux measurements showing average light-saturated (1500 μmol m^-2 s^-1 photon flux densities) summer daytime NEE of 16 μmol CO₂ m^-2 s^-1.  Among the three dominant oak species at the site, Quercus  velutina consistently had the highest light-saturated photosynthetic rates (22 μmol CO₂ m^-2 s^-1) and Q. coccinia had the lowest maximum rates (18 μmol CO₂ m^-2 s^-1), with Q. prinus performing at an intermediate position (20 μmol CO₂ m^-2 s^-1) as measured with a LiCor 6400. Canopy position-specific 4CA results also agreed with leaf-level photosynthetic chamber measurements at these positions, which further confirmed model results. During a dry spell in August 2006, carbon assimilation dropped to 3 g C m^-2 day^-1 for the entire stand indicating dry soil conditions induced strong stomatal limitations to productivity at the site.  In addition, annual carbon uptake modeled with 4CA was in good agreement with annual biomass production estimates which was augmented with seasonal soil respiration measurements