# Fire Activity and Fuel Consumption Dynamics in Sub-Saharan Africa
## 4 Results
### MOD17 Productivity Assessment
This may result from grass and litter comprising the largest fraction of combusted fuel in African savanna fires, with relatively little of the woody material being consumed by comparison [73; 74]. The fuel moisture content of these fine fuels is also typically low during the peak fire season, which leads to high (a mean of \(\approx\)0.93) combustion completeness [73; 74; 36; 75]. The fire clusters in this analysis were selected on the basis that they burned in consecutive years, and this frequent fire occurrence may help explain the similarity in the relationship between fuel consumption and integrated PSN between land cover types. Savanna fuel loads, principally grasses, tend to be greater in areas where fires occur on a biennial or longer timescale due to fuel build up (e.g., in moribund and unpalatable grass), which is influenced by the fire return interval, precipitation patterns, productivity and grazing pressure [61]. Large areas of Africa burn frequently (Figure 9) and the majority (81%) of the pixels in the fire clusters used in this analysis burned every three years or fewer between 2005 and 2016. Differences in fire occurrence may lead to variations in  
Figure 8: Relationship between FRE-derived fuel consumption (Tg DM) and accumulated MODIS PSN (Tg) on a fire cluster basis as a function of land cover type in northern (**a**) and southern (**b**) hemisphere Africa. The grey shaded area represents the prediction interval and the blue shaded area is the slopes 95% confidence interval. Note that the \(x\)- and \(y\)-axis range differs among plots. fuel build up within each fire cluster, since the accumulated PSN only accounts for the contribution of productive vegetation through fPAR measurements. Variations in the build-up of senescent fuel are not represented, and could increase the variability between fuel consumption and accumulated PSN measures. The similarity in the proportion of fuel consumed relative to the accumulated productivity as a function of land cover type is reflected in Figure 10, which presents the FRE-derived fuel consumption per unit area (kg DM m\({}^{-2}\)) and the integrated PSN (kg m\({}^{-2}\)) for northern (Figure 10a,b) and southern (Figure 10c,d) hemisphere Africa. The upper and lower bounds of the bars depict the 25 th and 75 th percentiles and the black line is the 50 th percentile. As with many natural phenomena, the fuel consumption estimates are best described by a power law where the 25 th percentile is close to the highest frequency. The average fuel consumption estimates are approximately 0.1 and 0.2 kg DM m\({}^{-2}\) at 25 th and 50 th percentiles respectively. Savanna, grassland and croplands show similar fuel consumption estimates, which lends support to the notion that herbaceous fuel is the primary material burned. In the southern hemisphere, the MODIS PSN estimates for forest, savanna and croplands are higher than the fuel consumption estimates at all percentile ranges, whilst shrublands and grasslands are lower in most cases. The FRE-derived fuel consumption estimates should be lower than the accumulated PSN since combustion completeness is rarely 100% and the PSN estimates contain contributions from woody respiration and maintenance and below ground biomass. The average accumulated PSN estimates are 1.32 and 1.65 (kg m\({}^{-2}\)) at 25 th and 50 th percentiles respectively and are greater than those of the northern hemisphere. The FRE-derived fuel consumption estimates for savanna, which is the dominant land cover type, is 89% and 70% lower at the 25 th and 75 th percentiles, respectively, which is close to the slope shown in Figure 8.  
Fuel consumption per unit area estimates for dambo grassland (0.22-0.29 kg DM m\({}^{-2}\)), savanna (0.35 kg DM m\({}^{-2}\)) and miombo woodland (0.42-0.45 kg DM m\({}^{-2}\)) contained in references [27; 74; 76; 77] are typically around twice those of the FRE-derived estimates.