Soil heating during the complete combustion of mega-logs and broadcast burning in central Oregon USA pumice soils

The environmental effect of extreme soil heating, such as occurs with the complete combustion of large downed wood during wildfires, is a post-fire management concern to forest managers. To address this knowledge gap, we stacked logs to create ‘mega-log’ burning conditions and compared the temperature, duration and penetration of the soil heat pulse in nine high intensity burned (HB) plots paired with adjacent masticated and broadcast burned low intensity burned (LB) plots at different soil depths (0, 5, 10 and 30 cm) in a Pinus ponderosa stand with volcanic pumice soils. Maximum soil surface temperatures ranges were 424–1168°C with a mean and standard error of 759 ± 9°C in the HB treatment and 42–360°C (107 ± 43°C) in the LB treatment. In the HB treatment, temperatures causing fine root and soil organism mortality (>60°C) penetrated the soil to at least 10 cm, but were not recorded at 30 cm. In the HB treatment, mean duration above 60°C at 0–10 cm persisted for 4–13 h (7.61 ± 1.02 h). Soils in the LB treatment experienced lethal temperatures at the surface for about an hour (1.19 ± 0.70 h) and at 5 cm were mostly well below lethal temperatures with the exception of one at 57°C and another at 100°C that remained above 60°C for 1.4 h. Large areas of high burn severity may affect long-term forest productivity. Our quantification of soil heating establishes conditions for ongoing studies investigating the effects of soil burn severity on tree seedling growth, soil fungi and nutrients.