Green roofs as habitats for wild plant species in urban landscapes: First insights from a large-scale sampling

The urban landscape is known to form a hostile matrix for wild plant communities because of its sealed infrastructure surfaces. However, green roofs can reduce this hostility by providing new spaces for wildlife directly on buildings. In this extensive study of 115 green roofs in northern France, we focused on wild plant communities and the variables that shaped their diversity and their taxonomic and functional compositions. A total of 176 colonizing vascular plant species were identified; 86% were natives, demonstrating that green roofs can serve as habitats for wild biodiversity despite their isolation in an urban landscape in three dimensions. Nonetheless, all types of green roofs were not equal, with the substrate depth playing a major role in the wild plant diversity. The taxonomic and functional compositions of the colonizing plant communities were also shaped by the substrate depth, green roof age, surface area, and height and maintenance intensity at the building scale. We did not detect any effect of the surrounding potential habitats at the landscape scale. The study of functional traits revealed that the wild plant communities are adapted to open xero-thermophilous conditions. This study led us to consider an ecological typology for green roofs referred to as stratum classification, which is based on the vegetal structures living and colonizing these anthropo-ecosystems. Wild roofs adapted to receive spontaneous species could play an interesting role in urban biodiversity dynamics if they continue to be developed at large scales in cities.