Carbon Stocks of Cameroon Savanna and Changes 2007-2010 using ALOS PALSAR Data
Mermoz, Stephane¹; Le Toan, Thuy¹; Villard, Ludovic¹; Maxime, Réjou-Méchain²; Joerg, Seifert-Granzin^3
1CESBIO, FRANCE; ²LEDB, FRANCE; ³Mesa-Consult, GERMANY

Recently, it was recognized that Africa is a major source of interannual variability in global atmospheric CO2 (Williams et al., 2007), although Africa contributes less than 4 % of the global fossil fuel emissions (Canadell et al., 2009). In Africa, savannas cover roughly 50 % of the continent and the main savanna vegetation types are woodlands, tree and shrub savannas, forest and savanna mosaic. In the context of the Reduced Emissions from Deforestation and forest Degradation (REDD+) programmes in Africa, emphasis has been put on biomass change in tropical rain forests, in particular in the Congo basin, whereas the studies mentioned in Ciais et al. (2011) pointed out the need to consider the contribution of savanna in the overall carbon budget when determining the direction of the African carbon budget (sink or source). Changes in african savannas can result in rapid changes in the overall carbon balance.

We have developed a methodology for the retrieval of savanna aboveground biomass (AGB) at national scale in Cameroon. The methods are based on ALOS-PALSAR and in situ data. SAR data acquired in 2007, 2008, 2009 and 2010 were processed with an emphasis on the reduction of uncertainties due to speckle, while preserving the SAR resolution (25 m). A regression model was developed with a reduced number of fitting parameters. The PALSAR mosaic data over Cameroon have been inverted into AGB and AGB uncertainties over 259 227 km² of Cameroon savanna for the four years. To do so, dense forests have been masked using global land cover maps such as Globcover 2009 (Bontemps et al., 2011). The multi-temporal 25 m resolution biomass maps of Cameroon have been exploited by quantifying 1) the carbon stocks of Cameroon savanna as compared to previous studies based on in-situ biomass estimates over the Congo basin, 2) changes in savanna biomass since 2007 and 3) correlations between biomass spatial distribution and environmental indicators such as climate, types of soils and topography.

We obtained a total AGB of 1253 ± 42.6 Mt or 626.4 ± 21.3 Mt in above ground carbon for Cameroon Savanna. The national assessment of savanna carbon stocks has been compared with results from prior studies (Nasi et al., 2009; Gibbs et al., 2007; Achard et al., 2004), showing significant differences between methods. For example, mean AGB over mosaic forest-savanna is much higher from radar than from results in Nasi et al. (75.6 versus 28 Mg/ha). This class with a surface area of 51 879 km² contributes significantly to the carbon budget. Changes in savanna biomass since 2007 have been mapped at 25 m resolution by using change detection algorithms over forests of the entire Cameroon. Two methods are tested. The first approach uses the ratio of the local means in the neighborhood of each pixel. Secondly, in order to consider highest-order statistics, we chose a probability density function (PDF) into the Pearson system using the estimated moments in the neighborhood of each pixel, then estimated the parameters of the distribution, and finally computed distances between multi-temporal PDF. Biomass changes in savannas are found to be largely affected by anthropogenic disturbance such as clearance for grazing, agriculture and particularly timber, especially around settlements.

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