Impact of plant choice and water management on the provision of ecosystem services by green roofs

Green roofs (GRs) can provide multiple ecosystem services (ESs) such as air quality improvement, biodiversity support and climate regulation. The hypothesis was tested that plants with different structural and functional characteristics (canopy size and density, evapotranspiration (ET) rates) differ in the provision of stormwater management and runoff quality improvement. Experiments were conducted comparing the potential of industrial standard GR cover (Sedum) to alternative species with higher ET rates (Heuchera micrantha ‘Obsidian’, Salvia officinalis and Stachys byzantina). High ET was strongly linked to rainfall retention, with Salvia and Stachys providing the greatest overall retention (32% of the total rainfall), although large, dense canopies were also advantageous when antecedent substrate moisture was high, with Sedum retaining 17% compared to 13% with Stachys. Species with high ET rates will require irrigation to maintain plant health and provision of ESs, but strategies for sustainable irrigation of GRs are needed. In this study, crop coefficients of 1.47, 2.98, 2.94 and 1.66 for Heuchera, Salvia, Stachys and Sedum respectively were calculated and used in a simple ET-based model (using the FAO-56 Penman-Monteith equation) that was proposed for scheduling GR irrigation based on plant water use. Greywater as an alternative water source for irrigation was also investigated. Plant health and functioning (and thus provision of ESs) for most species were not affected by greywater irrigation with no negative effects in Stachys and Sedum, but ET rates of Heuchera and Salvia were reduced by 14% and 9% respectively after 6 weeks of greywater irrigation. Species differences were also apparent in improvement of runoff quality after both freshwater and greywater irrigation, with the highest quality runoff from Stachys. Results highlight the importance of appropriate species choice for improving the provision of ESs on GRs, and suggest that the irrigation requirements of these species could be met by using greywater.