South-east Asia's tropical rainforests are experiencing the highest rate of deforestation worldwide and fire is one of the most important drivers of forest loss and subsequent carbon dioxide emissions. In this study, we analyzed all fire events in Borneo recorded by satellites over a period of 10 years. About 16.2 Mha, which corresponds to 21% of the land surface, have been affected by fire at least once and 6% more than one time. During El Nino conditions, which cause prolonged droughts in the region, the fire-affected area was on average three times larger than during normal weather conditions. Similarly, fires in forests affected 0.3 Mha in normal years and 1 Mha during El Nino years. Carbon rich peat swamp forest ecosystems were most severely affected. There is a pronounced difference in fire occurrence between different countries and provinces in Borneo although ecosystem and land use are very similar across the island. Compared with Sarawak, Sabah (Malaysia) and Brunei the relative annual fire-affected area in Kalimantan, the Indonesian part of Borneo, was on average five times larger. During El Nino conditions the fire-affected area increased only in Kalimantan and not in Brunei and the Malaysia. A similar pattern was observed in National Parks. This suggests, that El Nino related droughts are not the only cause of increased fire occurrence and do not necessarily lead to a higher number of fire events. These results improve our understanding of existing fire regimes and drivers of fire in SE Asian tropical ecosystems and may help to better protect the remaining rainforests.
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