Soil heating during wildfires and prescribed burns: a global evaluation

Background: Fires can alter soil properties via downward heat transfer. Numerous studies have examined effects of wildfires and prescribed burns on soils, yet knowledge of the soil temperatures and durations reached is limited. This can lead to erroneous assumptions regarding fire impacts, especially when laboratory heating results are extrapolated to field conditions. Aims and methods: We compiled new and published data on maximum temperatures and heating durations for mineral soils during wildfires and prescribed burns in forests, shrublands and grasslands around the globe, and compared these with data from laboratory heating experiments. Key results: Most fires heated only the uppermost centimetres of the soil, rarely exceeding 300°C below 0.5 cm depth. Where 300°C was exceeded at the soil surface, heat pulses were shorter (<500 s) than those often applied in laboratory studies (30 min to 1 h). The highest soil-surface temperature occurred in a shrubland wildfire (~964°C), and longest heating durations in forests with deep duff layers (>3 h above 60°C). Conclusions and implications: Most fires, except in deep smouldering fuels, generate short and shallow soil heating. Laboratory studies with long heating durations rarely represent field conditions. When investigating fire effects on soil, inclusion of shallow near-surface layer samples is recommended.