Wildfire impacts on soil hydrological properties and erosion in stony and shallow soils of a forested Mediterranean catchment in Italy

Wildfires can affect agroforest-ecosystems generating several cascade effects on the soil-water continuum, among which erosion is one of the most important. Even so, few field studies analyse post-fire soil erosion at watershed scale, especially in the Mediterranean Basin, although here wildfires are particularly frequent.

The present work analyses the impact of the first rain events following one severe wildfire in the Pisan Mountains, Tuscany region, Italy, with the aim to quantify and characterize the post-fire impact in terms of soil erosion and sediment yield at watershed scale.

The study site is characterized by olive groves, maquis, maritime pine and chestnut forest, depending on the elevation. The area was affected by a fire involving an area of about 1000 ha in September 2018. Fire severity was assessed by remote sensing imagery, while fire impact on soil properties and sediment yield was assessed by field experiments, and sampling and lab analysis. Finally, a hydrological model was implemented in HEC-HMS environment for exploring the relationship between the erosion-deposition events monitored in a sample watershed, and the hydrological processes induced by the rainfall events.

Experimental analysis revealed that the organic fraction of topsoil dramatically declined after the wildfire, together with the saturated hydraulic conductivity. Fire occurrence also generated a shift in the soil water repellency. The analysis of sediment deposited at the outlet of the catchment revealed that the first two rainfall-induced erosion-deposition events transported a larger amount of non-organic sediments, probably already close to the riverbed before the fire, while the other four events analyzed corresponded to deposited material much richer in organic matter, produced by the fire event. Overall, at watershed scale, soil erosion amounted to 7.85 t/ha, a relatively moderate rate considering the large fire impact on soil. This could be partly explained by the shallow soil depth of the study area (around 50 cm on average) and the high stoniness of soil, which would lead to preferential infiltration pathways for water, reducing runoff and erosion.