Allometric models and aboveground biomass stocks of a West African Sudan Savannah watershed in Benin

Background: The estimation of forest biomass changes due to land-use change is of significant importance for estimates of the global carbon budget. The accuracy of biomass density maps depends on the availability of reliable allometric models used in combination with data derived from satellites images and forest inventory data. To reduce the uncertainty in estimates of carbon emissions resulting from deforestation and forest degradation, better information on allometric equations and the spatial distribution of aboveground biomass stocks in each land use/land cover (LULC) class is needed for the different ecological zones. Such information has been sparse for the West African Sudan Savannah zone. This paper provides new data and results for this important zone. The analysis combines satellite images and locally derived allometric models based on non-destructive measurements to estimate aboveground biomass stocks at the watershed level in the Sudan Savannah zone in Benin. Results: We compared three types of empirically fitted allometric models of varying model complexity with respect to the number of input parameters that are easy to measure at the ground: model type I based only on the diameter at breast height (DBH), type II which used DBH and tree height and model type III which used DBH, tree height and wood density as predictors. While for most LULC classes model III outperformed the other models even the simple model I showed a good performance. The estimated mean dry biomass density values and attached standard error for the different LULC class were 3.28 +/- 0.31 (for cropland and fallow), 3.62 +/- 0.36 (for Savanna grassland), 4.86 +/- 1.03 (for Settlements), 14.05 +/- 0.72 (for Shrub savanna), 45.29 +/- 2.51 (for Savanna Woodland), 46.06 +/- 14.40 (for Agroforestry), 94.58 +/- 4.98 (for riparian forest and woodland), 162 +/- 64.88 (for Tectona grandis plantations), 179.62 +/- 57.61 (for Azadirachta indica plantations), 25.17 +/- 7.46 (for Gmelina arborea plantations), to 204.92 +/- 57.69 (for Eucalyptus grandis plantations) Mg ha(-1). The higher uncertainty of agroforestry system and plantations is due to the variance in age which affects biomass stocks. Conclusion: The results from this study help to close the existing knowledge gap with respect to biomass allometric models at the watershed level and the estimation of aboveground biomass stocks in each LULC in the Sudan Savannah in West Africa. The use of model type I, which relies only on the easy to measure DBH, seems justified since it performed almost as good as the more complex model types II and III. The work provided useful data on wood density of the main species of the Sudan Savannah zone, the related local derived biomass expansion factor and the biomass density in each LULC class that would be an indispensable information tool for carbon accounting programme related to the implementation of the Kyoto Protocol and REDD+ (reducing emissions from deforestation and forest degradation, and forests conservation, sustainable management of forests, and enhancement of forest carbon stocks) initiatives.