CARBON UPTAKE BY SECONDARY FORESTS IN BRAZILIAN AMAZONIA

Estimating the contribution of deforestation to greenhouse gas emissio ns requires calculations of the uptake of carbon by the vegetation tha t replaces the forest, as well as the emissions from burning and decay of forest biomass and from altered emissions and uptakes by the soil. The role of regeneration in offsetting emissions from deforestation i n the Brazilian Legal Amazon has sometimes been exaggerated. Unlike ma ny other tropical areas, cattle pasture (rather than shifting cultivat ion) usually replaces forest in Brazilian Amazonia. Degraded cattle pa stures regenerate secondary forests more slowly than do fallows in shi fting cultivation systems, leading to lower uptake of carbon. The calc ulations presented here indicate that in 1990 the 410 X 10(3) km(2) de forested landscape was taking up 29 X 10(6) t of carbon (C) annually ( 0.7 t C ha(-1) year(-1)). This does not include the emissions from cle aring of secondary forests, which in 1990 released an estimated 27 X 1 0(6) t C, almost completely offsetting the uptake from the landscape. Were the present land-use change processes to continue, carbon uptake would rise to 365 X 10(6) t annually (0.9 t C ha(-1) year(-1)) in 2090 in the 3.9 X 10(6) km(2) area that would have been deforested by that year. The 1990 rate of emissions from deforestation in the region gre atly exceeded the uptake from regrowth of replacement vegetation.