Soil surface changes increase runoff and erosion risk after a low–moderate severity fire

Abstract

Post-fire land degradation is to a large degree determined by what happens to soil properties and ground cover during and after the fire. To study fire impact in relation to fire intensity and post-fire soil exposure, a 9-ha Portuguese shrubland catchment was burned by experimental fire in the 2008/9 winter season. Previous studies reported on the significant increase in erosion after this fire, and discussed the role of reduced canopy interception and changed soil water repellency dynamics. Our objective was to assess whether fire-induced changes in soil physical properties and soil surface characteristics may have played an additional role in the increase in runoff and erosion observed after the fire. We sampled these properties before, immediately after, and up to one year after fire, and monitored soil temperatures during the fire using thermocouples. Despite the locally high fire intensity (>15.000kWm−1 in some places), soil physical changes were not observed: topsoil bulk density, organic matter, porosity and saturated conductivity did not significantly change, likely because soil temperatures stayed low with the 0.5cm depth not exceeding 32.5°C. Soil surface characteristics did change: Manning's n and random roughness both decreased, increasing the risk and erosivity of overland flow. Results indicate that soil physical changes unlikely contributed to the increase in post-fire erosion observed in the catchment and that a high-intensity winter burn does not necessarily lead to severe soil changes. Nevertheless, soil surface changes during and after fire contribute to an increase runoff and erosion risk in these areas.