Wildfire effects on soil erodibility of woodlands in NW Spain

Abstract

Knowledge of soil erodibility following wildfire is of crucial importance for prioritisation of post‐fire restoration practices for soil erosion mitigation. The present work therefore aims to determine the effect of wildfire on soil erodibility for common woodlands in Galicia, NW Spain. This is done by comparing selected topsoil properties of 28 pairs of recently wildfire‐burned and neighbouring unburned sites on different geologic substrates. The soil properties were selected for their supposed importance in erodibility, and include aggregate size distribution and water aggregate stability. Comparison of the burned and unburned sites suggested that wildfire had a noticeable negative effect on aggregate size distribution but not on particle size distribution. The effect on aggregate stability was highly variable. Aggregate stability was clearly lower at the burned than unburned site in about a third of the cases and in the remaining cases either basically the same at both sites or higher at the unburned site. The differences in aggregate stability, like those in aggregate size distribution, appear to be associated with changes in organic carbon content. The impact of wildfire on soil erodibility is supposed to operate through its effect on soil organic matter and, thus, to depend strongly on fire severity. Soil erodibility is then little affected by low‐severity wildfires but markedly diminished following high‐severity wildfires. All burned topsoils were strongly to very strongly water repellent. Fire‐enhaced repellency is therefore viewed as a key factor in the post‐fire runoff and erosion processes repeatedly observed in recently burned areas in NW Spain.