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

Abstract

1. 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.2. 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.3. 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. 4. 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.

Canopy plant composition and structure of Cape subtropical dune thicket are predicted by the levels of fire exposure

“…Our results on compositional differences along a fire exposure gradient are broadly consistent with those from other systems where fire-sensitive and fire-dependent systems coexist in the same landscape. The degree of fire exposure is an important influence in the composition of vegetation in forest-fynbos systems in the CFR ( Manders, 1990 ; Geldenhuys, 1994 ) and in the tropical forest-savanna systems of Africa, Brazil and Australia ( Charles-Dominique et al, 2015 ; Hoffmann et al, 2009 ; Murphy & Bowman, 2012 ; Flake et al, 2021a ; Flake et al, 2021b ; Becket et al, 2022 ). In both systems, regular fires maintain the more open, fire-dependent ecosystems (fynbos, savanna) by preventing invasion of fire-sensitive forest species ( Geldenhuys, 1994 ; Hoffmann et al, 2009 ).…”

“…Climatically and edaphically homogeneous landscapes may support structurally and floristically distinct biomes, raising the question as to what determines the boundaries between them ( Bond, Midgley & Woodward, 2003 ; Hoffmann et al, 2012 ; Butler et al, 2014 ; Coetzee, Bond & Wigley, 2015 ; Cowling & Potts, 2015 ; Cramer et al, 2019 ). In landscapes dominated by fire-prone vegetation such as savannas, grasslands and heathlands, fire regime effects have been invoked to explain the boundaries between fire-prone and fire-avoiding biomes, for example Afrotemperate forest patches in Cape fynbos shrublands ( Manders, 1990 ; Geldenhuys, 1994 ; Cowling & Potts, 2015 ; Bond, Midgley & Woodward, 2003 ) and rainforest patches in savanna and grassland ( Hoffmann et al, 2009 ; Murphy & Bowman, 2012 ; Becket et al, 2022 ). The coastal dunes of the Cape Floristic Region (CFR) are an interesting case in this regard, since they can support three biomes, namely dune forest, subtropical dune thicket (hereafter “dune thicket”) and dune fynbos, each of which experiences different fire regimes as determined by topographically-induced fire protection and species-specific flammability properties ( Pierce & Cowling, 1991 ; Cowling & Potts, 2015 ; Msweli et al, 2020 ; Cowling & Hoffman, 2021 ).…”

“…The effects of fire regimes on vegetation composition and structure have been studied extensively in fire-prone vegetation across the globe ( e.g. , Bond & van Wilgen, 1996 ; Williams, Gill & Bradstock, 2012 ; Rundel et al, 2018 ; Becket et al, 2022 ). Even though dune thicket is not a fire-dependent vegetation type, it experiences fire and is resilient to high fire severity as most, if not all, thicket canopy species are capable of resprouting post-fire from dormant basal buds or epicormic buds in the canopy ( Cowling & Pierce, 1988 ; Strydom et al, 2020 ).…”

“…Several studies have investigated the influence and importance of fire on species composition and structure of savanna-forest mosaics of Africa, Australia and Brazil ( Charles-Dominique et al, 2015 ; Hoffmann et al, 2009 ; Murphy & Bowman, 2012 ; Flake et al, 2021a ; Flake et al, 2021b ; Becket et al, 2022 ). In these systems, regular fires promote savanna tree and grass abundance and suppress forest trees, which are mostly killed by fire; however, when the fire frequency increases, savanna trees can also be suppressed, thereby favoring grasslands.…”

“…In these systems, regular fires promote savanna tree and grass abundance and suppress forest trees, which are mostly killed by fire; however, when the fire frequency increases, savanna trees can also be suppressed, thereby favoring grasslands. Accordingly, if fire is absent for too long, forest tree abundance will increase, and savanna tree and grass cover will decrease ( Hoffmann et al, 2009 ; Flake et al, 2021b ; Becket et al, 2022 ). Less is known about the influence of fire on species composition and structure of closed-canopy vegetation types that rarely burn, such as Afrotemperate forest ( Watson & Cameron, 2001 ; Giddey, Baard & Kraaij, 2022a ) and thicket ( Cowling, 1984 ; Vlok, Euston-Brown & Cowling, 2003 ; Cowling & Potts, 2015 ); however, some information on species-specific responses to fire is available, which leads to the question of how fire exposure shapes community composition in these closed communities.…”