Rehabilitation of forest soils affected by wildfires

Abstract

Fires are one of the most frequent and important disturbances of forest ecosystems. When, after a severe fire, heavy rainfall occurs in a burnt area of pronounced relief, there is a potential risk of strong alterations in the hydrological behavior of the affected basins. This can lead to large increases in surface runoff and intense erosive episodes (Vega, Fernández and Fontúrbel, 2018). These phenomena favor soil degradation and can cause floods and landslides, threatening human life, infrastructure and various valuable resources within and outside the burnt area. Traditionally, hydrological-forestry restoration after fires has been mainly focused on the recovery of the destroyed vegetation cover and on the reduction of soil and sediment losses after fire that, in most of the environments, occurred during the first year (Fernández and Vega, 2016b). Therefore, it is necessary to apply emergency soil stabilization measures to try to reduce: a) runoff and soil erosion risk and consequently, the maintenance/conservation of the quality of water and aquatic habitats and b) the degradation of soil, which is an essential element for the recovery of the affected ecosystem. These measures favor indirectly the maintenance/recovery of most physical, chemical, biochemical and microbiological soil properties, which are related to the soil organic carbon (SOC) stock and hence to the maintenance/conservation of the quality of the burnt soil. Given the risk of large-scale hydrological events, these actions allow, above all, to protect human life and a set of valuable resources that can be critically threatened in a very short time after the fire events. To this end, their objectives are to protect the burnt soil, to limit its disintegration and subsequent loss of C and nutrient stocks, uprooting and transport, as well as to reduce runoff, while stabilizing the watercourses, where appropriate. The most efficient treatment to achieve that objective is the application of a mulching of different plant materials (straw, wood strands, wood chips) over the burnt soil surface (see the reports by Robichaud, Beyers and Neary, 2000, Vega et al., 2013a and Fernández et al., 2019a). This reproduces the natural conditions in a pine forest affected by a low or medium severity wildfire, when the fall of pine needles forms a mulch that protects the soil from erosion; in this case, the implementation of these emergency soil stabilization measures is not necessary.