Top-down models and flux measurements are complementary methods of estimating carbon sequestration by forests: illustrations using the 3-PG model.

Abstract

AbstractFluxes measured by eddy covariance and calculated using process-based ecosystem models must be consistent with integrated values of carbon sequestration obtained from biomass-inventory data. Ecosystem models provide the tools needed for spatial extrapolation from flux measurements and to evaluate the impacts of past, current and future alterations in the environment. This chapter illustrates the use of a simple process-based model (3-PG; Landsberg and Waring, 1997) as a tool to bridge the gap between flux and physiological measurements and inventory measurements. The structure and characteristics of the model are outlined and an example is provided of the accuracy with which it can be calibrated against biomass measurements made in an experiment in Australia where treatments resulted in widely differing nutrient status. The results illustrate the need to account for nutrition in estimating carbon sequestration. A comparison of several ecosystem models with flux measurements in Oregon is carried out, and the points emerging from the comparisons are discussed. The 3-PG model was used to simulate growth of a Sitka spruce stand in Scotland for which estimates of net ecosystem exchange (NEE) were available from integrated flux measurements. The implications of the assumptions that had to be made to estimate NEE from the model output are discussed. We note progress in the use of measurements made from satellites to account for variability in conditions affecting rates of carbon exchange and sequestration.