Turned to Ashes: the Environmental Costs of Using Fire in Slash-and-burn Agriculture Outweigh Its Benefits

Fire has been a fundamental tool for human evolution and well-being. In slash-and-burn agriculture (SBA), it has been used for millennia to eliminate field debris, accelerate the incorporation of nutrients into the soil, and control weeds. However, in addition to these desired outcomes, fire can have multiple undesired environmental impacts that threaten the sustainability of tropical forests and SBA itself. We hypothesized that the environmental costs of fire use in SBA can outweigh its agronomic benefits across tropical forests. To assess this hypothesis, we critically reviewed empirical evidence on both desired and undesired environmental effects of SBA-related fire across the tropics worldwide. We found that fire effectively facilitates land clearing and releases nutrient-rich ashes that temporarily fertilize the soil. However, this fertilization is short-lived and restricted to the topsoil, while a substantial fraction of forest nutrients is wasted through volatilization and post-fire erosion. SBA-related carbon emissions may also contribute to climate forcing at regional and global scales. Although fire can reduce weed pressure by damaging seeds and limiting seed bank viability, this recruitment limitation can delay or arrest secondary succession, thereby constraining forest recovery and its ability to offset fire damage in SBA-dominated landscapes. In addition, repeated burning increases the risk of accidental wildfires, amplifying forest loss and associated biodiversity loss and carbon emissions. Elevated soil temperatures during burning can negatively impact soil biota, disrupting their essential ecological functions. Overall, despite notable remaining knowledge gaps, our review suggests that the environmental costs of using fire in SBA outweigh its benefits.