Traditional roofs cannot efficiently infiltrate these storm water accumulations and they increase the land surface temperature causing urban heat island effects. More importantly, especially increased impervious surface areas are one of the most important factors posing risk of flooding downstream and increasing storm water storage. In recent years, extensively growing environmental concerns in urban areas have led to creating contemporary solutions which provide improved storm water management, regulation of buildings’ temperatures, and reduction in air pollution. It is concluded that: (a) to create resilient green roofs, spontaneous colonisation should be accepted and considered as a design factor and (b) regional plant communities could serve as a model for seed recruitment and installations. In terms of plant life strategies, the competitive species sown on the roof gradually gave way to stress-tolerant and ruderal species, along with a progressive increase in species with shortdistance seed dispersal strategies. The main driver of the observed functional changes was a shift towards relatively more thermoxeric conditions. In contrast, from 1999 to 2014 species diversity remained steady, suggesting that nestedness prevailed over species turnover. Results demonstrated that from 1987 to 1999 the species diversity increased and the species turnover prevailed over nestedness, due to the progressive niche occupation by new species. Three sets of 120 phytosociological relevés, sampled in 1987, 19, have been compared to assess: (1) nestedness vs spatial turnover, (2) functional diversity and (3) the importance of vegetation dynamics on green roof performance and design. The roofs were greened in 1985 with soil-based turf rolls sown with a mixture of five grasses (Festuca rubra, Festuca ovina, Agrostis capillaris, Lolium perenne and Poa pratensis). The variations in species composition and assemblage of unmanaged simple-intensive green roofs in Hannover, Germany, were investigated over a thirty year period, in order to assess the persistence of the initial seed mixture and to evaluate floristic changes.
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