Fire in sub‐Saharan Africa: The fuel,cure and connectivity hypothesis

Aim

Past analyses of satellite‐based fire activity in tropical savannas support the intermediate fire–productivity hypothesis (IFP), which posits a close correlation with estimates of total net primary productivity in drier savannas and declines towards the extremes. However, these analyses ignore the distinct roles played by herbaceous and woody vegetation in fire ignition and spread. We hypothesize that, as herbaceous vegetation provides the primary fuel, fire activity in African savannas is asymptotically correlated with herbaceous production. Conversely, woody production affects fires indirectly through effects on herbaceous production and its connectivity. In contrast to the IFP, we propose the fuel, cure and connectivity (FCC) conceptual model for tropical fire activity. The FCC model makes explicit the distinct role of herbaceous and woody fuels, avoiding the confounding interpretation of the role of total production, while providing opportunities to quantify fuel curability, effects of trees on herbaceous fuel growth and connectivity, and human management.

Location

Sub‐Saharan Africa (SSA).

Time period

2003–2015.

Major taxa studied

Woody and herbaceous vegetation.

Methods

We used boosted regression tree analysis to test competing models explaining fire activity: (a) aggregate fuel loads; and (b) partitioned woody and herbaceous fuel loads; both derived from MODIS leaf area index.

Results

Herbaceous fuel load was consistently most influential, providing more explanatory power than overall biomass in fire activity. Fuel curability rated second, then human population density (HPD), and woody biomass was least important. We observed an asymptotic relationship between herbaceous fuel load and fire activity consistent with the FCC model; trees promote fires at low densites but suppress fires at higher densities; fires were rare in wetter regions, emphasizing the need for fuel to cure; and fires were concentrated in areas of low human population, underscoring the crucial role of land management.

Conclusions

The proposed FCC framework provides a more nuanced understanding of fire activity in tropical ecosystems, where herbaceous biomass is the key determinant of fire activity.