Near-complete loss of fire-resistant primary tropical forest cover in Sumatra and Kalimantan

Deforestation in Sumatra and Kalimantan in recent decades has made increasingly large parts of the region vulnerable to fires. Burning is particularly widespread in deforested peatlands, leading to globally significant carbon emissions. Here we assess primary forest cover loss and fragmentation combined with active fire observations between 2001 and 2019 for the entire region. We find that fires did not penetrate undisturbed primary forest areas deeper than two kilometres from the forest edge irrespective of drought conditions, highlighting the resistance of such forests to burning. However, only 10% of primary forest on peatland and 13% on non-peatlands remain in this category, with the rest being severely fragmented or degraded due to proximity to the forest edge. Fire-resistant forests now cover only 3% of peatlands and 4.5% of non-peatlands in Sumatra and Kalimantan. We also show that 15% and 17% of burning during the 2015 and 2019 fire episodes respectively occurred on land cleared since 2001. Our work demonstrates that protection and regeneration of the remaining contiguous primary forest blocks, as well as peatland restoration, are an urgent necessity for mitigating the impacts of potentially more frequent fire events under future global warming in Indonesia