Pathways of forest savannization in a mesic African savanna-forest mosaic

Abstract

O_LIFires 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. C_LIO_LIHowever, 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. C_LIO_LIHere, 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 between from forest to savanna. C_LIO_LIWe 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. C_LIO_LIWhile thickets had less tree mortality in fires than forests, repeat fires nonetheless precipitated a transition towards savannas. C_LIO_LIColonization of the savanna tree community lagged behind the grass community, but also began to transition. C_LI SynthesisOur 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.