Vegetation cover and biodiversity levels are driven by backfilling material in quarry restoration

In limestone quarries, after quarry abandonment, ecological restoration takes place subsequently over several years, often with the use of different procedures and backfilling materials. The success of the different restoration actions performed at a limestone quarry (Colle Pedrino, Lombardy Prealps, Italy) in terms of vegetation cover and biodiversity levels, compared to the surrounding natural areas, was evaluated in this work. Taking into account the soil's chemical-physical characteristics, 24 vegetation plots of 3 × 3 m were set up on seven restored parcels of different ages inside the quarry, of which one was left to spontaneous succession, and one control area outside the quarry was also considered for comparisons. Linear mixed models were employed to assess the effect of the following environmental variables on restoration: age, and several soil factors (stoniness, sand content, CN, K, and organic matter). The restoration success at the different parcels, in terms of total plant cover, grass cover and α- and β-diversity (including Lepidoptera), were thus determined. The variations in plant traits associated with the main environmental factors and floristic composition at the restored parcels were also assessed. The linear models revealed that the grass cover strongly depends on the age of the parcel and some soil factors: sand and potassium contents in the topsoil and organic matter in the subsoil. Moreover, plant and butterfly richness depend on the soil's stoniness and sand content. The species traits also significantly explained the variation in species distribution along the environmental gradients: stronger correlations with sand, potassium contents and stoniness were found. The restoration success in the quarry slopes depended on the backfilling materials used for slope reclamation over time. This factor concurs with the time passed since the restoration/abandonment and has a key background role in shaping the quarry's grass cover and biodiversity gradients. Regarding species assemblages, the key leaf traits of plants linked to resource availability (leaf area, leaf nitrogen and leaf carbon contents) consistently responded to the soil variables within parcels. In conclusion, site-specific studies to evaluate the grain size and the chemical composition of the backfilling materials used in the slope remodelling are recommended to favour a resource-driven plant establishment, spontaneous recolonisation, higher biodiversity and slope stability.