Comparison of mining spoils to determine the best substrate for rehabilitating limestone quarries by favoring native grassland species over invasive plants

Abstract

Habitats being restored in Belgian quarries are easily invaded by non-native plant species, which can hamper the germination and development of vegetation deemed to be of high conservation value. Substrates of terraces created when mining limestone quarries could be inhospitable to native plants. However, they can provide opportunities for establishing specific vegetation, such as dry calcareous grasslands. Applying suitable mining spoils could be a cost-effective way to provide growing substrate when restoring limestone terraces. We assessed the efficacy of using mining spoils, collected on-site, as a potential growing substrate (bedding material). We tested gravely limestone (product of on-site mining activities), limestone dust (by-product), and no addition (bare limestone bedrock) to determine which was best for favoring the growth of native, dry calcareous grassland species and discourage the growth of two non-native invasive species that commonly invade altered mining sites: Buddleja davidii Franch and Senecio inaequidens DC. In a field experiment (in two quarries), we studied short-term (2 y) growth response of native and invasive species after sowing three seed mixtures of native grassland species, varying in functional diversity (and one no-sowing control treatment), all treatments subjected to competitive pressure exerted by invasive species. Percent cover of native and invasive species, species abundance and reproductive characteristics of the invasive species were monitored during 2-y. Native grasslands coverage was low on all substrate types, demonstrating how slowly calcareous grasslands species establish in such harsh substrate conditions. However, type of substrate did show a significant relationship with plant abundance, with limestone dust being the most beneficial for native species establishment (coverage). Although limestone dust appeared to be the best option for restoring grassland species to limestone quarries (based on its low cost, wide availability, and potential to support native species), it was also likely to support the two invasive species. Functional diversity of the seed mixture had no consistent effect. Our study shows the importance of identifying the most appropriate substrate to both establish calcareous grasslands and resist invasive species. This approach provides insights into developing strategies to conserve biodiversity in industrial and agricultural landscapes with limestone quarries.