Wildfire burn severity and post-wildfire time impact mechanical and hydraulic properties of forest soils

Vegetation contributes to the overall slope stability and is recognized as an environmentally friendly nature-based solution. Wildfires burn and denude vegetated slopes, thus increasing the risk of shallow landslides and debris flows. However, little attention has been given to assessing the effects of burn severity and the time elapsed since a wildfire on the hydro-geomechanical properties of burned slopes. This study performed a series of standard laboratory tests to evaluate the shear strength and saturated hydraulic conductivity (k_sat) of forest soils collected from moderate-low (ML) burned, moderate-high (MH) burned, and unburned (UB) test plots. The plots were sampled one, four, and six months after the March 2022 wildfire in Uljin County, South Korea. The results show that the continuous deterioration of roots highly depended on the burn severity. The root biomass of ML- and MH-burned soils was consistently lower than those of the UB soils. The root deterioration reduced the shear strength of the soils temporally. The burned soil's cohesion intercept was 1.80–2.30 times lower than that of the UB soil six months post-wildfire, with the friction angle unaffected. One- and four-months post-wildfire, k_sat of the burned soils was 1.22–3.15 times lower than the UB soil. Such lowered k_sat was due to the fine ash-clogged pores and hydrophobic layers beneath the soil surface. However, six months post-wildfire, the burned soils' k_sat increased by approximately twice that of the pre-wildfire condition because of macropore flow passages formed by impoverished roots. The appreciation of sand fraction, depreciation of fines content, and weakening of hydrophobicity over time have also emphasized their role in the temporal shifts in the properties of the ML- and MH-burned soils. The documented results herein can be incorporated into rainfall infiltration and stability analyses of wildfire-affected slopes, landslide susceptibility mapping, and mitigation measures design.