Functional difference in plant communities as a driver of green roof ecosystem services

Abstract

Green roofs provide ecosystem services that help humans in urban environments. Ecosystem services provided by green roofs include reducing storm water run-off, better regulation of building temperatures, reduced heat flux and urban heat island effect and providing a place for wildlife to inhabit. This study aims to expand the knowledge of how plant species functional traits and the plant community's functional diversity determine the ecosystem services green roofs provide. The experimental design compared seven species mixture treatments that differed in functional diversity, comprised of 11 plant species, with monoculture treatments. Nine replicates of each treatment were planted in trays and established on a roof in Halifax, NS, in a block design accounting for shading from an adjacent building. This study examined 10 response variables that indicate ecosystem services and used regression to assess the ability of functional diversity in the mixtures to predict ecosystem services. There was a very weak negative relationship between functional diversity and two response variables: canopy density and floral abundance. Position on the roof relative to shade showed that areas more sheltered by an adjacent building had greater stability in summer soil temperature regardless of functional diversity. While functional diversity was not positively related to any of the ecosystem services, several of the species mixture treatments outperformed the best monoculture treatments. Additional research on functional divergence as a predictor for green roof ecosystem services needs to be conducted.