IMPACT OF DROUGHT STRESS AND OTHER FACTORS ON SEASONAL LAND BIOSPHERE CO2 EXCHANGE STUDIED THROUGH AN ATMOSPHERIC TRACER TRANSPORT MODEL

We develop a simple, generic global model of carbon cycling by terrestrial vegetation driven by climate data, observed greenness from global vegetation index data, and a drought-stress indicator calculated with a one-layer bucket model. Modelled CO2 fluxes are then fed into a three-dimensional atmospheric tracer transport model, so that results can be checked against observed concentrations of CO2 at various monitoring sites. By exploring a wide range of model formulations, we find an optimal fit of the model to atmospheric CO2 data. To check these results, we use a second model with a coupled photosynthesis-hydrology scheme, using the Penman-Monteith equation to calculate evapotranspiration. This model simulates feedbacks between drought-stress and photosynthesis through the soil-water balance. We show that CO2 measurements at tropical and southern-hemisphere stations can be used to constrain the seasonal atmosphere-biosphere carbon exchange in the wet-dry tropics. In both models, this seasonality is strongly suppressed, more strongly in fact than predicted by some complex terrestrial-vegetation models. We also find some evidence of a considerable CO2 release from soils during the northern-hemisphere winter. An exponential air-temperature dependence of soil release with a Q(10) of 1.5 is found to be most appropriate, with no cutoff at low freezing temperatures. The results of this study should indicate in how far measurements of atmospheric CO2 concentration can provide a constraint on global models of terrestrial-vegetation activity.