Impact of burn severity of the July 2022 forest fire on soil hydrophobicity in the Elbe Sandstone temperate forest

Soil water repellency is a common phenomenon often associated with wild fires, which leads to a temporal change of forest ecosystems, for example, by enhanced overland flow, soil erosion and limited plant growth. Forest fires are expected to play an increasingly larger role due to climate change, resulting in more frequent droughts, higher temperatures, heatwaves, and landcover changes in temperate latitudes. Despite that importance, only a few studies have been published concerning soil water repellency in temperate European forests, and relatively little is known about soil hydrophobicity associated with so far rare forest fires in Central European spruce and beech forests.

In this study, we examine the impact of different burn severities on soil hydrophobicity down to 15 cm below the surface in the National Park “Sächsische Schweiz” after the forest fire in summer 2022, using the Water Drop Penetration Time (WDPT) test. Measurements were limited to the conductivity or non-conductivity of the water, with the test terminated at a time of 900 s. Various parameters, that could control water conductivity were examined, including burn severity from drone data, ground vegetation, duff layer, slope angle, slope aspect, and elevation of the site. In addition, soil properties such as soil type, carbon and nitrogen contents were analysed. We find a high spatial variability of hydrophobic plots in the studied area. The most hydrophobic plots were found in low severity sites rather than in moderate-high to high severity sites. Plots lacking a duff layer were more likely to exhibit hydrophobic layers. Soil water repellency was also found in unburnt sites. No distinct correlation was found between slope angle, slope aspect, elevation and the occurrence of hydrophobic plots. Plots located in coniferous forests exhibited higher frequencies of hydrophobicity compared to deciduous forests. That large variability and non-agreement with typically formulated relationships argue for a need to rethink the transferability of assumptions from traditional fire regions such as the Mediterranean or the boreal zone to the emerging fire regimes of temperate forests under climate change, requiring more empirical data.