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Article Summary1. Fires in savannas limit tree cover, thereby promoting flammable grass accumulation and fueling 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. 2. 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. 3. 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 between from forest to savanna. 4. 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. 5. While thickets had less tree mortality in fires than forests, repeat fires nonetheless precipitated a transition towards savannas. 6. 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. Files:Composition Data.csvTree composition data for each transect, species split by size classesgrass_comp_biomass.csv Grass composition data for each transectPlot_cover_fires_sampling.xlsxInformation about transect sampling dates, historical tree cover based on Beckett & Bond (2019), and fire history of each transect

Typically, savannas experience frequent fires, which limit tree cover and promote flammable grass accumulation, whereas forests and thickets form dense canopies that exclude fires by reducing C4-grass fuel loads and creating a humid microclimate. However, extreme fires occasionally burn into these closed canopy systems. Although these are known to cause tree mortality and can make repeat fires more likely, the long-term consequences of an extreme fire event for closed canopy vegetation structure and potential savannization remain largely unknown. Here, we analysed whether an extreme fire event could, alone, alter species composition, vegetation structure, and fire regimes of closed canopy ecosystems in a natural savanna-forest-thicket mosaic, or whether successive fires after an initial extreme fire event were necessary to trigger a biome transition between these alternative stable states. 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. Thickets had lower levels of tree mortality following fires than forests, although repeat fires also led to a transition towards open grassy systems, structurally resembling savannas. Our results suggest that while extreme fires set the stage for a biome transition, subsequent fires are necessary for grass composition to resemble neighbouring savanna grass communities, ultimately resulting in a savanna fire regime. Savannization, following extreme fire events, is possible in natural mosaic ecosystems where open and closed canopy vegetation types represent alternative stable states.

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Disease emergence events regularly result from human activities such as agriculture, which frequently brings large populations of genetically uniform hosts into contact with potential pathogens. Although viruses cause nearly 50% of emerging plant diseases, there is little systematic information about virus distribution across agro‐ecological interfaces and large gaps in understanding of virus diversity in nature. Here we applied a novel landscape‐scale geometagenomics approach to examine relationships between agricultural land use and distributions of plant‐associated viruses in two Mediterranean‐climate biodiversity hotspots (Western Cape region of South Africa and Rhône river delta region of France). In total, we analysed 1725 geo‐referenced plant samples collected over two years from 4.5 km x 4.5 km grids spanning farmlands and adjacent uncultivated vegetation. We found substantial virus prevalence (25.8–35.9%) in all ecosystems, but prevalence and identified family‐level virus diversity were greatest in cultivated areas, with some virus families displaying strong agricultural associations. Our survey revealed 94 previously unknown virus species, primarily from uncultivated plants. This is the first effort to systematically evaluate plant‐associated viromes across broad agro‐ecological interfaces. Our findings indicate that agriculture substantially influences plant virus distributions and highlight the extent of current ignorance about the diversity and roles of viruses in nature.