Fire incongruities can explain widespread landscape degradation in Madagascar's forests and grasslands

Societal Impact Statement The relationship between rainfall, fire and habitat can display incongruencies. The 2021 Malagasy Grassy Biomes Workshop identified understanding fire regimes as a knowledge gap. This study pinpoints regions where anthropogenic fire has the potential to transform or has transformed habitat to treeless‐grassland, by identifying landscape‐scale, island‐wide fire anomalies across half of Madagascar. Its eastern forests burn like savannas, and central‐western grasslands burn frequently and intensely despite receiving rainfall usually associated with forest and fire‐absence. Recognising the incongruity and better understanding its drivers can mitigate against landscape‐scale degradation, improving ecological function, and human well‐being. Summary Data show that since 1953, human‐lit fires on Madagascar have transformed clear‐cut forest to treeless‐grasslands. To address the extent of Malagasy treeless‐grasslands at human settlement, the 2021 Malagasy Grassy Biomes Workshop identified the role of fire as a critical knowledge‐gap for understanding ecological function. The relationship between mean annual precipitation (MAP), fire and habitat is well established across mesic systems. Anthropogenically transformed habitats often deviate from expected ecological patterns, so we tested for landscape‐scale, island‐wide MAP‐related fire and habitat anomalies. We collated Malagasy fire, habitat and MAP datasets, identifying location and scale of incongruities relative to global fire‐habitat‐MAP expectations. Next, we tested for mismatches in fire regimes (frequency, timing, extent and intensity of fires) between Malagasy and equivalent global biomes, using global, comprehensive landscape‐scale fire regime data. Across half of Madagascar, fire frequency and habitat are decoupled from MAP, and fire regimes across Malagasy ecoregions differ significantly from those in shared biomes elsewhere in the world. Landscape‐scale incongruities span Malagasy eastern forests (which burn like savanna systems) and central‐western treeless‐grasslands, which burn frequently and intensely despite receiving MAP typical of forest presence and fire‐absence, globally. Fire‐MAP incongruities identify potentially transformed areas, or those undergoing transformation by fire, and establish a platform for investigating the nuanced social, political and ecological dynamics that may contribute to and perpetuate these anomalies. Incongruities also highlight the anthropogenic landscape degradation associated with fire anomalies. Addressing these impacts can facilitate restoration of ecological function, productivity and food security, benefiting biodiversity and humans at multiple scales.