EXTRAPOLATING LEAF CO2 EXCHANGE TO THE CANOPY - A GENERALIZED-MODEL OF FOREST PHOTOSYNTHESIS COMPARED WITH MEASUREMENTS BY EDDY-CORRELATION

Over the last 4 years, two data sets have emerged which allow increase d accuracy and resolution in the definition and validation of a photos ynthesis model for whole forest canopies. The first is a greatly exp e xpanded set of data on the nitrogen-photosynthesis relationship for te mperate and tropical woody species, The second is a unique set of long -term (4 year) daily carbon balance measurements at the Harvard Forest , Petersham, Massachusetts, collected by the eddy-correlation techniqu e. A model (PnET-Day) is presented which is derived directly from, and validated against, these data sets. The PnET-Day model uses foliar ni trogen concentration to calculate maximum instantaneous rates of gross and net photosynthesis which are then reduced for suboptimal temperat ure, photosynthetically active radiation (PAR), and vapor pressure def icit (VPD). Predicted daily gross photosynthesis is closely related to gross carbon exchange at the Harvard Forest as determined by eddy cor relation measurements. Predictions made by the full canopy model were significantly better than those produced by a multiple linear regressi on model. Sensitivity analyses for this model for a deciduous broad-le aved forest showed results to be much more sensitive to parameters rel ated to maximum leaf-level photosynthetic rate (A(max)) than to those related to light, temperature, VPD or total foliar mass. Aggregation a nalyses suggest that using monthly mean climatic data to drive the can opy model will give results similar to those achieved by averaging dai ly eddy correlation measurements of gross carbon exchange (GCE).