Plant trait analysis delivers an extensive list of potential green roof species for Mediterranean France

Abstract

Green roofs are increasingly popular in urban areas of NW Europe and North America. However, greening roofs still needs incentives particularly in countries with a Mediterranean climate. Although green roof benefits such as cooling capacity and storm water retention would be even more pronounced in this challenging climate with hot and dry summers, vegetation stress on extensive green roofs will be enhanced, causing low performance if the same species as in the temperate regions of NW Europe and North America are used. In order to support further development and application of green roofs in the Mediterranean climate, new insights on suitable native species is an essential step. Using the habitat template concept, specifically taking into account drought adaptation and self-regulation, we developed a screening procedure using both functional plant traits and utilitarian aspects. Plant traits of two species lists (one resulting from an extensive vegetation study and another one covering the successful plant species of extensive green roofs in NW Europe) were analyzed. The results were incorporated into a hierarchical multi-criteria screening tool. This tool can encourage further experimental trials and inspire and guide the green roof industry toward the most appropriate species for extensive green roof design. As an example, the key was illustrated on a subset of plant species from Mediterranean southern France, which identified 34 newly potential green roof species. Interestingly, 35% of these species were annuals, a promising life form that has until now rarely been considered for extensive green roofs.