Soil thickness and porosity as indicators of the ecological restoration success: The case study of a reclaimed coal-mine slope in a Mediterranean area

Abstract

Open-cast mine ecological restoration is challenging due to the total removal of vegetation and soil. Thus, restoring soil functionality is a key goal to underpin long-term ecosystem resilience. Understanding soil prop- erties change during the revegetation process is essential for evaluating restoration success efforts and guiding adaptive management based on reliable soil indicators. We assessed two vegetation patches representing distinct successional stages –grassland (pioneer) and shrubland (mature)– on the same mine slope restored eleven years prior. Within each patch, 18 plots (3 transects × 6 sampling units) were established to analyse topography, plant family cover, and soil physicochemical properties. Soil thickness and porosity emerged as the most explicative indicators (20 % and 17 %, respectively) for vegetation cover variance. These were also strongly associated with both functional soil recovery indicators (C/N ratio, cation exchange capacity, available water) and vegetation progression indicators (Fabaceae and Poaceae %cover), based on structural equation modeling and principal component analysis. Fabaceae and other families, typically associated with late-successional stages, were linked to low porosity and deeper soils, while Poaceae and Asteraceae, indicative of early successional stages, were associated with high porosity and shallow soils. We propose soil thickness and porosity as cost-effective and easily measurable indicators for monitoring ecological restoration on post-mining slopes, as they reflect both soil recovery and vegetation dynamics. We also recommend their inclusion in restoration monitoring protocols to support adaptive management and improve alignment with international ecological restoration standards.