Climate change and deforestation boost post-fire grass invasion of Amazonian forests

Interactions among climate change, deforestation and fires are changing the stability of the Amazon forest, and may promote transitions to degraded grassy ecosystem states. However, our ability to predict the locations in the Amazon that are most vulnerable to these transitions is limited. In this study we used a dynamic carbon model to evaluate how drought, climate change and deforestation could affect the probability of post-fire grass invasion across the Amazon, and identify where grass-fire feedbacks may promote the persistence of species-poor degraded forests with savanna-like structure. Our results suggest that, under current climatic conditions, post-fire grass invasion could affect 11% of the Amazon, with the south-eastern Amazon at highest risk of invasion. We forecast that under business as usual climate change, by the end of the century areas with a high probability of post-fire grass invasion will increase to 20% of the Amazon. In 10% of the Amazon fire return interval will be shorter than the time required for canopy recovery, implying high risk of irreversible shifts to a fire-maintained degraded ecosystem state. Although resilience in canopy regeneration is evident in areas with low fire frequency, increased fire frequency could inhibit regeneration even in forests where grass is currently excluded, and push the Amazon forests towards a tipping point causing large areas of forest to transition to low tree cover state.