Fire in the rainforest: A 3200‐year history of fire in a West Kalimantan, Indonesia, tropical rainforest

Abstract

AbstractDespite its perceived historical rarity, fire is an important disturbance in tropical rainforests. Very large rainforest fires have been observed multiple times in recent decades, often during years of strong El Niño‐Southern Oscillation droughts. Fire in rainforest has major short‐term consequences for humans and wildlife by converting forest to fire‐prone fern, shrub, and grass, but the long‐term effects remain to be seen. Borneo's indigenous groups have been using fire to clear land for centuries, yet the prevalence and spatial patterns of premodern fire across forest types in Borneo are not well understood. This research set out to reconstruct fire in a 1500‐ha primary rainforest spanning 800 m of elevation in Indonesian Borneo with the goal of elucidating the role humans have played in rainforest fire. We found that humans played an important role in the occurrence of fire in recent centuries. Evidence of fire—charcoal >2 mm—is more abundant in forest types where humans would be more likely to live and/or practice swidden agriculture. However, pyrogenic material is ubiquitous across the study area, showing that all forest types have experienced fire. A set of 50 radiocarbon dates showed that in lowland areas—where human‐caused fire is most likely—fire occurred throughout the last 3200 years, peaking 1300–1600 ce. The upland areas lacked evidence of fire before 1250 ce but otherwise had a similar pattern to the lowlands. The period of high fire coincides with regional demographic changes as well as regional droughts documented elsewhere in Southeast Asia. In upland areas, fires likely burned only under regional drought when fires could more easily spread upslope. Although forest plot studies at this site show little structural evidence of past fires, tree diversity is lower than expected in the most burned areas (alluvial benches). Thus, our results suggest that land clearance was a major source of fire, but the current intact state of these rainforests indicates that they were largely resilient to fires and land use hundreds of years ago. Recent fires mirror patterns of fire spread that occurred hundreds of years ago, though their severity and extent are likely much greater.