Pathways of savannization in a mesic African savanna-forest mosaic following an extreme fire

Fires in savannas limit tree cover, thereby promoting flammable grass accumulation and fuelling further frequent fires. Meanwhile, forests and thickets form dense canopies that reduce C4 grass fuel loads and creating a humid microclimate, thereby excluding fires under typical climatic conditions. However, extreme fires occasionally burn into these closed-canopy systems. Although these rare fires cause substantial tree mortality and can make repeat fires more likely, the long-term consequences of an extreme fire for closed-canopy vegetation structure and potential to convert to savanna (hereafter 'savannization') remain largely unknown. Here, we analysed whether an extreme fire could, alone, alter species composition, vegetation structure and fire regimes of closed-canopy ecosystems in an intact savanna-forest-thicket mosaic, or whether successive fires after an initial extreme fire were necessary to trigger a biome transition from forest to savanna. We found that forests that only burned once recovered, whereas those that burned again following an initial extreme fire transitioned from closed-canopy forests towards open, grassy savannas. While thickets had less tree mortality in fires than forests, repeat fires nonetheless precipitated a transition towards savannas. Colonization of the savanna tree community lagged behind the grass community, but also began to transition. Synthesis. Our results suggest that rare extreme fires, followed by repeated burning can indeed result in savannization in places where savanna and forest represent alternative stable states.