Known for their cooling effects, stormwater management capabilities and biodiversity support, green roofs are becoming increasingly important in urban areas around the globe. In a study published in Urban Ecosystems by lead author Dr Kelly Ksiazek-Mikenas and her team, the researchers compared the use of non-native Sedum species with native grass and forb communities on green roofs, exploring how these varied plant communities may enhance these urban installations’ ability to provide essential ecosystem services.
Dr. Ksiazek-Mikenas and colleagues uncovered that native plant communities, despite slower establishment, demonstrated higher rates of stormwater capture and evapotranspiration compared to their Sedum counterparts. Inversely, Sedum species exhibited superior thermal insulation capabilities during the sweltering months of summer. One surprising discovery was that indigenous arbuscular mycorrhizal fungi showed no significant influence on plant establishment or the provisioning of ecosystem services. The group’s findings underline the potential benefits in using a blend of native prairie grasses and forbs to optimize green roofs’ ecosystem services.
Green roofs are systems engineered to host plant life atop urban buildings—an innovative solution aimed at reducing urban heat island effect and managing stormwater runoff. Traditionally, succulent Sedum species—which are hardy and grow quickly— have been the plant of choice for these installations. The choice of plant species, however, directly affects the biodiversity and other ecosystem services provided by the green roofs.
The researchers employed green roof trays and employed a multi-factorial design to test vegetation community and arbuscular mycorrhizal fungi inoculation’s effects on ecosystem services provisioning. Native plants and Sedum species were subjected to a variety of urban environmental conditions. Measurements of stormwater capture, thermal insulation, and plant survival were taken over a span of time to compare the plant communities’ performance.
The findings revealed that native prairie species absorbed more stormwater than the non-native succulents – an important factor in reducing flooding in urban areas. They also had higher rates of evapotranspiration than Sedum species, implying that they can help restore the soil to a dry state more swiftly after rain events and contribute towards local cooling.
Despite their slower establishment, the use of native plant species thus presents several benefits towards stormwater management and localized cooling. However, Sedum still outperformed the native species in providing thermal insulation during the hottest months, reinforcing the value of plant diversity in green roof design.
The research proposes a more diversified approach in choosing plant species for green roofs. In their article, and colleagues write that a mix of native plant species and Sedum can potentially improve their performance in managing stormwater, promoting local cooling, and providing thermal insulation.
Although it is possible to mimic natural habitats on green roofs (Ksiazek-Mikenas et al. 2021), we were not able to maintain prairie species cover in the experimental trays so that they realistically represented natural communities…
In the event that mimicking both below- and above-ground natural communities is not feasible in green roof installations, we suggest establishing a community with as much functional diversity as possible. For example, both Allium cernuum and Opuntia humifusa could be planted alongside Sedum and, with their variation in growth and bloom times, could provide ecosystem services for a greater portion of the temperate growing season. Additional summer cooling, prolonged evapotranspiration and increased stormwater capture may all be provided by adding native forbs and grasses if the species are able to survive and maintain cover over time,Ksiazek-Mikenas et al. 2023
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Ksiazek-Mikenas, K., Chaudhary, V.B. and Skogen, K.A. (2023) “Combinations of plant species with complementary traits have the potential to maximize ecosystem services on green roofs,” Urban Ecosystems. Available at: https://doi.org/10.1007/s11252-023-01383-3.
Cover image: Green roofs by Canva.