Exploring the use of spectral indices to assess alterations in soil properties in pine stands affected by crown fire in Spain

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, 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. We 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. The findings demonstrate the potential applicability of remote sensing to determining changes in soil properties after fire.