Effects of wildfire on soils: field studies and modelling on induced underground temperature variations

Wildfire statistics report an increasing number of deaths, damages, and force society to face exceptional repair costs also due to secondary effects such as landslides, soils erosion and water quality problems. Literature agrees that the extent of the thermal disturbance of the soil due to wildfire is strongly dependent on fire intensity, on its duration and recurrence, as well as on fuel load, and soil properties. The unpredictability of these phenomena is one of the main challenges for scientists trying to study their characteristics. Degradation of the biological, chemical, and physical properties of forest soils due to the wildfires can reduce their capacity to function fully, with such effects either temporary or permanent. Direct measurements of the effects of these phenomena are difficult to be collected in the field, especially when the area affected by fires is a wilderness. In this study, controlled fires reproducing real conditions of wildfire affecting the Susa Valley (NW Italy) in 2017, which induced intense erosion and debris flows, have been monitored both above and below the ground surface. Moreover, numerical simulations based on real data increased the know-how to reproduce real changes on the underground. The small-scale fire simulations have allowed to verify how the depth affected by significant increases in temperatures is truly pellicular. Field tests show that at −2 cm depth temperature never exceed 70°C, meaning that it usually does not affect soil components or properties. The results of the study suggest that temperature variations in the subsoil are very localized and limited. If confirmed by further studies, the processes inferred to produce large ground effects on slopes after wildfires should be re-considered.