IN SEARCH OF THE MISSING CARBON SINK - A MODEL OF TERRESTRIAL BIOSPHERIC RESPONSE TO LAND-USE CHANGE AND ATMOSPHERIC CO2

Estimates of the net exchange of carbon between the terrestrial biosph ere and the atmosphere may be too large because the models of carbon r elease from changes in land use do not allow for enhanced carbon assim ilation by the terrestrial biosphere in response to increasing atmosph eric CO2. We address this deficiency with a model of terrestrial biosp here that includes both ecosystem response to land-use perturbation an d vegetation response to atmospheric CO2. Model inputs specify the are as affected by land-use change since 1700. The carbon dynamics of the affected areas are described by an area distribution function for vege tation carbon density and a compartment model of carbon in vegetation, litter, and soil. Vegetation growth is modeled as the difference betw een net primary production (NPP) and mortality. NPP, the net flux of c arbon from atmosphere to vegetation, is a logistic function of vegetat ion carbon density. The response of NPP to atmospheric CO2 is modeled with three response functions: a logarithmic, a rectangular-hyperbolic , and a response function derived from a biochemical model of C-3 phot osynthesis. The response functions are parameterized by ecosystem type with data from CO2 exposure experiments. Elevated CO2 affects the NPP of both undisturbed and recovering ecosystems. We use the model to te st the hypothesis that the CO2 enhancement of terrestrial NPP explains the historical missing carbon sink of the the global carbon cycle bud get. Our estimates of the biosphere's CO2 enhanced carbon flux are muc h smaller than the reconstructed missing carbon sink. We conclude that our model results do not support the hypothesis.