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

Abstract

Typically, savannas experience frequent fires, which limit tree cover and promote flammable grass accumulation, whereas forests form dense canopies that exclude fires by reducing C4-grass fuel loads and creating a humid microclimate. However, extreme fires occasionally burn into forests. Although these are known to kill forest trees and can make repeat fires more likely, the long-term consequences of an extreme fire event for forest structure and potential forest savannization remain largely unknown. Here, we analysed whether an extreme fire event could, alone, alter forest species composition, vegetation structure, and fire regimes, or whether successive fires after an initial extreme fire event were necessary to trigger a biome transition. We found that forests that only burned once recovered, whereas those that burned again following an initial extreme fire event underwent a transition from closed-canopy forests to open, grassy systems. Our results suggest that, while extreme fires set the stage for a biome transition, subsequent fires are necessary for flammable grasses to colonise and establish, ultimately resulting in a savanna fire regime.

Unifying deterministic and stochastic ecological dynamics via a landscape-flux approach

“…For a forest to establish, trees must first establish as saplings, whereas they need not regress into saplings before dying, such that the composition of the system is quite different as a forest opens up compared to when a savanna closes over (see ref. 48 for empirical evidence supporting this mathematical intuition).…”

Feedbacks in ecology and evolution

“…A strong reduction of these drivers (e.g., long fire-free interval during moist years, or herbivore exclusion or extinction) allows the colonization of forest trees that shade the environment (favouring forest trees; positive feedback) with the consequent switch to a closed biome [6,34]. Similarly, in a closed biome, a change in the drivers (e.g., a high intensity fire driven by extreme weather conditions) can open the canopy, increasing flammable grasses and fire, thus switching to an open biome [6,35]. The two negative feedbacks stabilise either a closed or an open biome in a given climate (bistability) and can remain as such for millennia [34].…”