Green Roofs as a Mainstreamed Nature-Based Solution Tackling the Challenge of Biodiversity Loss

Green roofs (GRs) belong to the third type of NbS, namely, the creation of new ecosystems synthetically defined as surfaces detached from the ground, spontaneously colonised by plants, or intentionally greened. Their strength lies in the multiple benefits (co-benefits) they offer to single buildings and the urban environment as a whole: from the absorption of air pollutants and the reduction of energy consumption in buildings to the provision of biodiversity. Concerning the latter service, if GRs are designed according to the principle of restoration ecology following the habitat template approach, they can play a key role as stepping stones to becoming part of the urban ecological network and the urban green infrastructure. Conceiving GRs as landscapes instead of flat homogeneous surfaces will improve biodiversity, for example, by varying substrate type and thickness, adding small temporary ponds and foreseeing areas with scattered vegetation. Among others, a way to select plant species and communities for green roofs is to take the phytosociological classification as a template given by the characteristic, diagnostic and recurrent species of natural stands. An accurate preliminary site analysis is essential to select the proper natural template and to replicate the edaphic conditions characterising it