Species transfer via topsoil translocation: lessons from two large Mediterranean restoration projects

Restoration success of species-rich grasslands is often limited by low seed dispersal. To reintroduce target species of local provenances, bulk topsoil transfer is performed by excavating soil and vegetation from donor sites and spreading these on receptor sites. Our first objective was to determine the most advantageous season for transferring topsoil in Mediterranean grasslands. The second objective was to assess which treatmentcombination of season (spring or autumn) and transfer ratio (1:1 or 1:3)-performed best in restoring a Mediterranean grassland through bulk topsoil transfer after two large restoration projects. Just-in-time soil transfer (i.e. with no stockpiling) was implemented: (1) on a former 3ha orchard where topsoil was spread after removing trees and leveling soil; and (2) after a 5ha oil pipeline leak where polluted soil was removed prior to treatment (soil horizons were reconstituted). A seed bank study showed that the summer seed bank contained higher seed densities, species richness, and similarity to the reference site than the spring seed bank. Spring transfers gave better results than autumn transfers in terms of species richness and composition similarity with the reference site, while transfer ratios gave similar results. Long-term success was not driven by season or transfer ratio but by the underlying seed bank at receptor sites: the former orchard's weed-containing seed bank hampered topsoil transfer success. This study also suggests that restoration success cannot be deduced from seed bank studies alone, as species establishment is highly dependent on differences in growing conditions (including competition at receptor sites).