Transférer le sol pour restaurer des communautés végétales: quelles leçons pour mesurer la résilience des pelouses sèches ? (Plaine de La Crau, Sud-Est de la France).

Abstract Ecological restoration can accelerate the resilience of degraded dry grasslands. Among the ecological restoration techniques used, soil transfer has already given promising results for restoring species-rich plant communities by providing the ability to recreate rapidly a habitat that corresponds to the reference ecosystem (the non disturbed dry grasslands). However, soil transfer is a destructive method, since it involves the use of non-renewable resources such as grassland soils of the donor sites that are the produce of centuries of interactions between climate, plants and animals. In south-eastern France, the plain of La Crau is an example of Mediterranean dry grasslands where the resilience of the steppe vegetation is extremely slow after land use changes which have impacted soil and vegetation. On 7th August 2009, a major oil leak occurred in the steppe center, destroying more than 13 acres of steppe vegetation. As a consequence, in 2010, the polluted soil was excavated and evacuated in a specialised dump. This operation was, then, combined with various in situ experiments of soil transfer, with exclusion of traditional sheep grazing management, to test (i) the importance of respecting the vertical organization of the main soil horizons and (ii) some opportunities to save this non-renewable resource. In May 2011, just after the soil transfer (72,000 tons) was achieved in April 2011 from a nearby quarry which extension had been authorised prior to the oil leak, different quadrats were materialized in the reference steppe vegetation around and in the restored site, with at the soil surface: the organic layer (top-soil, treatment ABC) with or without compaction (treatment ABCnc), the mineral layer (sub-soil, treatment BC) or the altered bedrock only (treatment C). After three years of vegetation monitoring, all the different treatments of soil transfer resulted in a rapid resilience of steppe vegetation in terms of floristic composition and plant species richness. Indeed, in 2013, the treatment with the transfer of mineral layer only at the soil surface, showed no significant difference in terms of plant species richness with the reference steppe plant community. Furthermore, there was a significant higher species richness for the treatments with the transfer of organic layer, compacted or not, in comparison with the reference steppe plant community. In addition, between 2011 and 2013, the natural colonisation by some ruderal opportunistic species was very low. Nevertheless, the reference steppe plant community organization was not restored for none of the different soil transfer treatments, as revealed by the calculation of the Bray-Curtis index. In addition, species abundances in the uncompacted organic layer, mean vegetation height and plant cover were significantly higher, than in the reference steppe plant community. Our results show that after only 3 years, the results of the different soil transfer treatments seem very promising. It appears from this three year monitoring that soil transfer of a mixture of the organic (top soil) and mineral (sub soil) layers, without final compaction, could be sufficient to boost the natural resilience of the typical plants of the reference steppe plant community. It might also be possible to increase the surface where the soil will be spread according to ratios of 1 to 2 or 1 to 3, instead of 1 to 1. This method will allow the protection of the steppe soil that is still a non-renewable resource. Because none of the different soil transfer treatments has allowed for the restoration of the integral steppe plant community, it seems now necessary to reinstall the traditional sheep grazing system with the aim to increase the restoration of the reference steppe plant community spatial organization. Résumé La restauration écologique peut accélérer la résilience des pelouses sèches dégradées. Parmi les techniques utilisées, le transfert de sol a, ainsi, déjà permis d’obtenir des résultats encourageants pour rétablir des communautés végétales riches en espèces. En revanche, il s’agit d’une méthode non durable, car destructrice du site donneur. Le 07 août 2009, une importante fuite d’un oléoduc est survenue au milieu de la steppe de La Crau. En 2010, plus de 5,5 ha de végétation ont été détruits par excavation et évacuation en décharge du sol pollué par les hydrocarbures. La restauration de ce site a, alors, été associée à différentes expérimentations in situ de remise en place du sol, en l’absence du pâturage ovin traditionnel (exclos), pour tester l’importance du respect de l’organisation verticale des principaux horizons pédologiques. Après trois années de suivi, nos premiers résultats montrent une forte résilience de la végétation en termes de composition et de richesse spécifique. De plus, peu d’espèces rudérales ont réussi à coloniser le site après transfert du sol. Cependant, aucune des modalités de remise en place du sol n’a pas permis de restaurer l’intégrité de la communauté végétale steppique de référence. Il semble, alors, maintenant nécessaire de remettre en place le système de pâturage ovin ancestral pour permettre la restauration de la structuration spatiale et de la diversité de la végétation steppique.