The severity of smouldering peat fires and damage to the forest soil

Smouldering wildfires propagate slowly through surface and subsurface organic layers of the forest ground and severely affect the soil, producing physical, chemical and biological changes. These effects are caused by the prolonged heating and the large loss of soil mass but are poorly documented in the literature. A series of smouldering experiments with boreal peat have been conducted under laboratory conditions to quantify these effects using small-scale samples. Peat samples of 100 mm by 100 mm in cross section and 50 mm in depth of different moisture were exposed to an external ignition source. Thermocouples placed throughout the sample bed measured the temperature evolution and tracked the peat ignition, intensity and spread of the smouldering front. The results show that moisture content controls peat ignition and that moisture below 125 ± 10% (in dry base) are required. The severity of the smouldering peat on the soil has been quantified in terms of temperature vs. residence time curves and mass loss. The measurements show temperatures in excess of 300 °C for residence times of 1 h leading to sterilization of the soil and mass loss in burnt layers above 90%.