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

Abstract

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 treatment—combination 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 3 ha orchard where topsoil was spread after removing trees and leveling soil; and (2) after a 5 ha 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).

A research agenda for seed‐trait functional ecology

“…Seed bank persistence is of particular importance to restoration projects that use topsoil as a seed source. A better understanding of the seed traits that enable seed persistence in stockpiled topsoil, and ways to manage the seed bank in receptor sites, will lead to improved restoration outcomes (Buisson et al, 2018).…”

Ecological succession in areas degraded by bauxite mining indicates successful use of topsoil

“…Some restoration attempts in open‐pit postmining areas have been published (Zhang et al 2001; Prach & Tolvanen 2016; Bulot et al 2017; Chenot et al 2017; Buisson et al 2018; Ribeiro et al 2018). However, it is still challenging to restore ecosystems with low resilience, such as those included in the OCBILs—the very old, climatically buffered, infertile landscapes.…”

Beyond the species pool: modification of species dispersal, establishment, and assembly by habitat restoration

“…Although everyone agrees that primary producers have the primacy in controlling the structuring of communities, predator–prey interactions also play an important role. Dense stands of exotic or ruderal species can reduce or enable native target species to establish (Buisson et al , ; Buisson et al ). Grazing is widely studied to restore open landscapes (e.g.…”

“…Buisson et al () evaluated the effects of the transfer ratio and the timing of topsoil transfer on the establishment of typical dry grassland species in Mediterranean grasslands. Although the seed bank contained more species in summer than in spring in their study, topsoil transfer in spring was more successful for the establishment of target species concerning both species richness and species composition compared to topsoil translocation in autumn.…”