RESPONSES IN NPP AND CARBON STORES OF THE NORTHERN BIOMES TO A CO2-INDUCED CLIMATIC-CHANGE, AS EVALUATED BY THE FRANKFURT BIOSPHERE MODEL (FBM)

To assess the role of the boreal and temperate forests and the tundra ecosystems in a future CO2-induced climate change, the Frankfurt biosp here model(FBM) was applied to the 3xCO(2) climate as calculated by th e GCM of the MPI fur Meteorologie in Hamburg. The FBM predicts on a 1 degrees x1 degrees spatial grid the seasonal and perannual course of l eaf biomass and feeder roots, woody biomass, soil carbon and soil wate r in response to the seasonal course of light, precipitation and tempe rature. The phenology is controlled by the flux balance of carbon gain s and losses, thus being dependent on the driving climate and the stat e of vegetation. Two equilibrium runs based on the 3xCO(2) climate wer e performed: (1) Considering the pure climate effect (with no direct C O2 fertilization) we obtained a 22% decrease of the net primary produc tion (NPP) due to enhanced autotrophic respiration and increased water limitation. Together with the effect on the soils this results in a 1 70 Gt carbon source. (2) Considering a CO2-induced enhancement of the maximum photosynthesis the pure climate effect is more than compensate d and we predict a NPP increase of 9% and a total carbon sink of 50 Ct C. This effect may even be an underestimate if one takes into conside ration a shift in the optimum temperature for photosynthesis under enh anced levels of atmospheric CO2 as proposed by Long and Drake.