State and change in carbon pools in the forests of tropical Africa

To improve estimates of the state and change in C pools due to changes in land use in tropical forests of Africa, we combined spatially explicit estimates of biomass C density, obtained by modelling in a geographical information system (GIS), with new data on the area of forests (woody formations with a minimum of 10% crown cover) reported at subnational units for 1980 and 1990 by the Food and Agriculture Organization (FAG). Estimates of the biomass C densities for grass/shrub savannas were also included using a simple model based on precipitation. The total C pool in above-and below-ground forests and grass/shrub savannas of Africa for 1980 was 50.8 Pg (10(15)g), with aboveground forest biomass accounting for 75% of the total, below-ground forest biomass for 21%, and grass/shrub savannas for 4%. Area weighted mean biomass C densities were about 180 Mg ha(-1) for lowland moist forests, 82 Mg ha(-1) for all forests, and 6 Mg ha(-1) for grass savannas. The total change in the aboveground forest C pool for the decade 1980-90 due to changes in land cover and use was estimated to be a decrease of 6.6 Pg C. Of this total, 43% was due to deforestation and 57% due to biomass reduction by other human activities. Six countries, mostly in central Africa, accounted for more than 73% of the total change in the C pool. The difference between state and change of C pool estimates made at the subnational scale and those made at the national scale proved to be insignificant across the region as a whole (2% for pools and -1% for change in pool) but potentially important to individual. countries (from +36% to -39% for pools and from +43% to -57% for change in pool). The differences between the two approaches may reflect a better match of the areas being deforested with the biomass C density of forests being cleared.