Abstract
BackgroundForest fires have increased in extent and intensity in the Mediterranean area in recent years, threatening forest ecosystems through loss of vegetation, changes in soil properties, and increased soil erosion rates, particularly in severely burned areas. However, establishing the relationships between burn severity and soil properties that determine infiltration remain challenging. Determining where soil burn severity evaluation should be carried out is critical for planning urgent measures to mitigate post-fire soil erosion. Although previous research has indicated that spectral indices are suitable for assessing fire severity, most of the classifications used consider combined effects in vegetation and soil. Moreover, the relationship between spectral indices and soil burn severity has scarcely been explored until now.ResultsWe selected three pine stands in Spain for study immediately after being burned by wildfires. We analyzed various soil properties (soil saturated hydraulic conductivity, mean weight diameter of soil aggregates, and soil organic carbon) in relation to six levels of soil burn severity in all three stands. In addition, we established 25 field plots in the burned areas. We computed ten spectral indices for each plot by using Sentinel-2 satellite data. The soil burn severity categories indicated the degree of degradation of important soil properties related to soil erosion susceptibility. Of the spectral indices considered, the relativized burn ratio (RBR) was the best predictor of cumulative infiltration and mean weight diameter of soil aggregates. The differenced mid-infrared bispectral index (dMIRBI) was most closely correlated with soil organic carbon content.ConclusionsThe findings demonstrate the potential applicability of remote sensing to determining changes in soil properties after fire.
Highlights
Forest fires have increased in extent and intensity in the Mediterranean area in recent years, threatening forest ecosystems through loss of vegetation, changes in soil properties, and increased soil erosion rates, in severely burned areas
This study explored the relationship between soil burn severity and spectral indices by considering three soil properties related to the susceptibility to soil erosion after fire: saturated hydraulic conductivity, as a surrogate for soil water repellency; soil organic carbon content; and the mean weight diameter of soil aggregates
Our study findings showed that the proposed categories of soil burn severity, based on visual signs, are useful for reflecting gradual changes in some soil variables that play a critical role in post-fire soil erosion susceptibility
Summary
Forest fires have increased in extent and intensity in the Mediterranean area in recent years, threatening forest ecosystems through loss of vegetation, changes in soil properties, and increased soil erosion rates, in severely burned areas. Determining where soil burn severity evaluation should be carried out is critical for planning urgent measures to mitigate post-fire soil erosion. Given the potentially large increases in overland flow events after wildfire and the associated flooding and erosion risk, accurate prediction and a good understanding of the relationships between soil properties that control infiltration and burn severity are essential, but remain challenging (Moody et al 2013). Some authors suggest the need for separate vegetation and soil burn severity assessment because these ecosystem components can be affected differently by wildfires depending on stand structure and fuel and fire behavior (Jain et al 2004; Jain and Graham 2007; Fernández et al 2020). Areas where soil burn severity should be assessed with a view to implementing urgent measures to mitigate post-fire soil erosion are those in which the vegetation is almost completely consumed during fire. Evaluation of post-fire soil burn severity still relies almost completely on field surveys because the relationships between spectral indices and soil burn severity have scarcely been explored until now (Marcos et al 2018; Sobrino et al 2019), and few attempts have been made to quantify alteration in soil properties using spectral indices (Moody et al 2016)
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