Estimates of net carbon exchange resulting from forest clearance and regrowth were made for three areas in the Brazilian and Bolivian Amazon. The study areas, ranging in size from 600 to 10 000 km2, include communities that practice a range of land uses from small-scale, rotational agriculture to long-term pasture. Carbon emissions from deforestation were estimated based on rates of deforestation derived from Landsat satellite data and published estimates of mature forest biomass. Estimates of carbon uptake by secondary forest regrowth were based on area estimates from the satellite data and biomass estimates from field surveys. Estimates of carbon emissions from the clearance of secondary forest were included. The total carbon emissions were partitioned into that immediately released by biomass burning during clearance and that from subsequent decomposition and burning of felled biomass. A range of net carbon fluxes from secondary forest regrowth and clearance was estimated by extrapolating land-use patterns in each study area to the entire area of deforested landscape in Amazonian Brazil and Bolivia, and by varying average biomass accumulation rate of regrowing forest and fallow period.
Over 60 km2 of forest were cleared from 1988 to 1995 in each of the Brazilian study areas, and over 940 km2 were cleared from 1986 to 1996 in the Bolivian study area. This resulted in 30–200 Mg C/yr emitted from biomass burning during deforestation. If the carbon that is committed to subsequent burning and decomposition of felled biomass is included, total committed emissions of 75–650 Mg C/yr resulted from deforestation in each study area. The carbon uptake by secondary forests was 33–435 Mg C/yr. However, emissions from the reclearance of secondary forests were nearly as large: 17–365 Mg C/yr. Extrapolation of these results to the Brazilian and Bolivian Amazon suggests that, at the national level, carbon uptake by secondary forests offset less than one-fifth of the carbon emissions from deforestation. Despite differences in land-use pattern, average fallow period, and rate of biomass accumulation used in the extrapolation, net carbon uptake by secondary forests was consistently small when compared to the total emissions from deforestation.