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Le città sono state paragonate da Odum a degli organismi eterotrofi che basano la loro crescita ed espansione sull’uso indiscriminato di risorse e sono causa di perdita irreversibile e frammentazione degli habitat naturali. I tetti verdi rappresentano uno strumento essenziale di mitigazione e compensazione ambientale all’interno del tessuto urbano dove, l’alta densità edilizia e l’elevato disturbo antropico concedono poco spazio alle dinamiche naturali. In particolare, i tetti verdi per la biodiversità caratterizzati da mosaici di micro-habitat diversi e contigui tra loro, possano ospitare specie con caratteristiche morfo-funzionali diverse. L’approccio noto come habitat template, consiste nel selezionare le specie vegetali adatte alle condizioni sui tetti estensivi tra quelle che in natura crescono in condizioni simili e.g. spessore di substrato ridotto e povero di sostanze nutritive, lunghi periodi di aridità. L’approccio fitosociologico prevede l’individuazione di habitat analoghi non solo come riferimento da cui estrapolare le specie vegetali, ma come modello per aggregare le piante in consorzi specifici, così come suggerisce l’interpretazione fitosociologica della natura. Cities are defined as heterotrophic organisms (Odum, 1983) as they consumes a huge amount of resources and causes habitat loss and fragmentation. Green roofs represents a fundamental mean of ecological compensation within the built environment, i.e. in highly altered and disturbed places by humans. In particular, green roofs for biodiversity (or biodiversity green roofs), being characterised by different but contiguous microhabitat (habitat mosaics or patches), can host several species with different functional traits. The methods known as habitat template consists of choosing suitable plant species for green roofs among the one that live in nature under similar conditions e.g. shallow and nutrient poor substrate, high drought. The phytosociological approach considers habitat analogue not only as species pools, but also as a model to group plants in specific associations as suggested in the phytosociological interpretation of nature.

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.