Global Validation of a Process-Based Model on Vegetation Gross Primary Production Using Eddy Covariance Observations

Gross Primary Production (GPP) is the largest flux in the global carbon cycle. However, large uncertainties in current global estimations persist. In this study, we examined the performance of a process-based model (Integrated BIosphere Simulator, IBIS) at 62 eddy covariance sites around the world. Our results indicated that the IBIS model explained 60% of the observed variation in daily GPP at all validation sites. Comparison with a satellite-based vegetation model (Eddy Covariance-Light Use Efficiency, EC-LUE) revealed that the IBIS simulations yielded comparable GPP results as the EC-LUE model. Global mean GPP estimated by the IBIS model was 107.50 +/- 1.37 Pg C year (-1) (mean value +/- standard deviation) across the vegetated area for the period 2000-2006, consistent with the results of the EC-LUE model (109.39 +/- 1.48 Pg C year (-1)). To evaluate the uncertainty introduced by the parameter V-cmax, which represents the maximum photosynthetic capacity, we inversed V-cmax using Markov Chain-Monte Carlo (MCMC) procedures. Using the inversed V-cmax values, the simulated global GPP increased by 16.5 Pg C year (-1), indicating that IBIS model is sensitive to V-cmax, and large uncertainty exists in model parameterization.