The combined effect of drainage density, land cover, and ENSO status on fire occurrence in tropical peatlands

Forested area has decreased while agricultural area and degraded lands have expanded in the Indonesian peatlands of Sumatra and Kalimantan over the past few decades. The expansion of agricultural lands followed by construction of drainage (canal) infrastructure affects peatland hydrology and fire risk. This study examined how drainage density affects fire occurrence (hotspot density) using the Visible Infrared Imaging Radiometer Suite dataset in the Indonesian peatlands of Sumatra and Kalimantan across a range of climatic conditions and land cover types. During 2013-2017, our study shows a greater hotspot density in peatlands with canals compared to undrained peatlands. Largest differences are found in forest by a factor of 13.3, followed by degraded land (7.6), cropland (5) and plantation (2.6) during the ENSO-neutral year of 2013. Hotspot density in low to moderately drained peatlands increases with drainage density with high correlation (r value 0.97, 0.96, 0.94, and 0.67 in forests, degraded lands, croplands and plantations respectively) in 2013. However, as El Niño commenced, the impact of drainage density on hotspot density weakened. Fires were widespread in undrained peatlands especially in the prolonged drought of the strong El Niño in 2015. In highly drained peatlands where plantations dominated, fewer fires occurred than on moderately drained peatlands, which could be explained by various factors including management practices and peat loss. A multiple regression model, incorporating factors of ENSO status, land cover types and drainage density explains 67% of the variability in hotspot density. Our results support the strategy of fire control and peat restoration, especially in unmanaged degraded peatlands.