The native - exotic plant choice in green roof design: Using a multicriteria decision framework to select plant tolerant species that foster beneficial arthropods

Abstract

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. So far, as green roofs begin to spread in South American cities, most technology comes from the northern hemisphere with little local information on native experiences. Moreover, decisions of using native or exotic plant species in green roofs had never been evaluated taking into account the plant ability to tolerate roof conditions together with their 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 Córdoba 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). In addition, we registered the number of interactions between selected plants and beneficial arthropods in half of the roofs, considering the influence of available flowers. Under watering and weeding, all selected species increased their vegetative cover one year after establishment. More interestingly, native plants with no management had an advantage over exotic plant species as they exhibited a significantly higher occurrence and a slightly higher cover than exotics. Native annuals were able to reseed the following the dry season even in the absence of management, thus highlighting the relative importance of lifespan as a useful plant trait for future studies in extensive green roof design. In addition, we showed that increasing flower availability in native plants promoted a higher number of interactions with pollinators and natural enemies; a response that was not observed for the exotic plant pool. Finally, by combining data on plants' cover and beneficial arthropods interactions we were able to validate the proposed ranking and selection procedure. 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 to tolerate roof conditions and promote beneficial arthropods would give a new twist in plant selection processes for green roofs.