Drainage density and land cover interact to affect fire occurrence in Indonesian peatlands

Fire occurrence in tropical peatlands is closely related to both land cover type and proximity to drainage (canal) networks. However, little is known about the extent to which land cover and drainage density interact to alter fire occurrence. Here, we assess the relationship between these variables in the peatlands of Sumatra and Kalimantan, Indonesia, spanning a five-year period of inter-annual climatic variability. Visible Infrared Imaging Radiometer Suite (VIIRS) imagery was used to map active fire hotspots. Drained peatlands experienced up to 13 times greater annual mean hotspot density (number of fire hotspots per km2) when compared to peatlands without canals. The greatest difference in fire hotspot density between drained and undrained peatlands occurred in forested peatlands (by a factor of 2.6-13.3), followed by shrublands (1.1-7.6), crop lands (1.4-5.0) and plantations (1.2-2.6), where largest differences were found in ENSO neutral years. We found a curvilinear relationship between hotspot density and canal density, with the relationship depending on land cover and ENSO status. At low to moderate drainage density, hotspot density increased with drainage density in all land cover types in 2013-2017. Heavily drained plantations experienced a lower hotspot density than moderately drained plantations possibly due to factors such as management practices or impacts of previous fire history. The relationship with drainage density was strongest in 2013, an ENSO-neutral year, and weakest in the strong El Niño of 2015. Our findings support the critical need for fire management in drained tropical peat areas. Peat fire management planning and peatland restoration should be tailored to the differing responses of fire to climate variability, drainage density and land cover types.