Restoring stone and dominant grass species cover in a Mediterranean grassland: 20-year effects on soil, vegetation, and arthropod communities

Long-term studies are needed to monitor recovery following restoration, as it may take decades or even centuries, particularly in the case of Mediterranean dry grasslands, for communities to reach their former equilibrium before the degradation occurred. A multi-component approach is also needed to evaluate restoration success and better understand the complex impacts of former restoration projects on present-day ecological interactions and ecosystem functions. The objective of this study was to address the restoration of a Mediterranean grassland in southern France, 20 years after implementation. Particularly, we examine the long-term impact on soil, vegetation, and arthropods of the reintroduction of a dominant grass species (Brachypodium retusum) and the restoration of the natural stone cover on a degraded former cultivated field. Soil analyses revealed that reestablishing stone cover only slightly acidified the soil, while B. retusum presence enhanced soil fertility. Brachypodium retusum also decreased the nutritional value of forage and increased plant biomass and litter. Plant composition shifted with treatments: B. retusum and stone cover favored xeromesophilous species, while B. retusum alone encouraged less palatable species. The soil seed bank composition and abundance were positively influenced by stone cover but negatively impacted by B. retusum reintroduction. Negative outcomes on some arthropods and mesofauna were measured in treatments with both B. retusum and stones, except for Acari, which were positively impacted by the presence of stones. The findings emphasize that B. retusum is an ecological engineer with complex effects on the different ecosystem components.