The native - exotic plant choice in green roof design: using a multicriteria decision framework to foster urban biodiversity

Green roofs are considered key elements of the urban green infrastructure since they offer several environmental benefits, including habitat provision for arthropods. To achieve these benefits and ensure green roof success, an appropriate plant selection is an important step in the design of these infrastructures, especially where green roof technology is emerging like in South American cities. So far, decisions of using native or exotic plant species in green roofs had never been evaluated taking into account the plant potential to foster beneficial arthropods. By applying an integrative multicriteria decision framework that combined the habitat template hypothesis with the potential of plants to attract floral visitors and natural enemies, we obtained a ranked set of candidate native and exotic plant species. Among the best-ranked candidate species, we further compared the performance of six native and six exotic species in 30 experimental green roofs installed in Cordoba city, Argentina. To evaluate plant success, the occurrence and cover of each species were recorded one year after establishment under two management conditions: regular watering and weeding of spontaneous plants, and no management (15 roofs each). All selected species increased their vegetative cover one year after establishment. More interestingly, native plants had an advantage over exotic plant species as they exhibited a significantly higher occurrence and a slightly higher cover with no management than exotics. Native annuals were able to reseed the following season even in the absence of management, thus highlighting the relative importance of lifespan as a useful plant trait for future studies in green roof design. Given that green roofs are one of the possible solutions to ameliorate the negative effects of urban habitat loss on arthropod diversity, the development of an integrative multicriteria decision framework that takes into account the potential of native and exotic plant species for promoting beneficial arthropods would give a new twist in plant selection processes for green roofs.