A plant sociological approach for extensive green roofs in Mediterranean areas

Extensive Green roofs can be an important mean for environmental mitigation if designed according to the principles of restoration ecology. Moreover, if optimally executed, properly managed and of sufficient extension, they could be assimilated to meta-populations of natural habitats, worth to be included in the biodiversity monitoring networks. The best example supporting this hypothesis is the Lake water plant Moos in Wollishofen (Zurich, Switzerland) where, on three 100 years old units of extensive green roofs, occur most of the typical flora of Mesobromion, including high density of some endangered orchid species. With this work, we propose a methodology approach for green roofs in Mediterranean areas, based on a practical plant sociology understanding of EU Directive 92/43: a recognition of Natura 2000 habitat that could be imitated on roofs in terms of characteristic species and substrates. Our results lead to three category groups: those linked to sandy substrates (psammophilous vegetation), to gravely-pebbly substrates (glareicolous vegetation) and to xeromorfic soils (garrigues and dry grasslands). According to the last theories and practical application for grasslands restoration, we suggest a method applied and studied in Switzerland for green roofs, based on diaspore hay transfer from a donor meadow, in order to obtain the highest plant species richness and diversity.