Phytosociological approach to implement a Sedum-dominated plant community in extensive Mediterranean green roofs in different N enriched substrates

Extensive green roofs (EGRs) provide some important ecosystem services including the increase of urban biodiversity. Besides Sedum, the most used genus, the establishment of plant species with different functionality increases the habitats. A survey of spontaneous plant communities, adapted to drought and low nutrients, can be a valid tool of enrichment for this urban green feature. In 2014, we carried out an experiment on a roof, setting up 12 plots of 10 cm depth filled with three different substrates composed of compost, pelletised paper sludge, and tephra (Vulcaflor), as follows: VC (Vulcaflor + compost), VPC (Vulcaflor + pellet + compost) and VP (Vulcaflor + pellet). Annual and perennial forbs, annual and perennial legumes, geophytes, CAM and grasses were planted and seeded in the boxes. After five years, the plant community was dominated by CAM (Sedum) and geophytes, and then, to increase the biodiversity, we carried out a seeding of species, mainly belonging to the phytosociological classes Stellarietea mediae and Koelerio-Corynephoretea. We monitored the total canopy cover, plant diversity (Shannon, Evenness and Simpson indices) and the percentage of plant functional types from October 2019 to June 2020. The different nitrogen contents (VC>VPC>VP) affected the spread of CAM in VPC and VP, allowing the annual seeded species to colonise. The plant diversity increased especially in VP substrate. Phytosociological approach resulted suitable to select plant species able to establish in extreme climate conditions, as well as to coexist with CAM species, increasing the diversity in poor nutrient substrate.