Across the country, cities are increasingly embracing green roofs due to their benefits. In 2018, the City of Portland mandated green roofs on buildings in the central city over 20,000 square feet.
According to Samantha Hartzell, assistant professor in the Department of Civil and Environmental Engineering at Portland State University, research shows green roofs increase stormwater retention, lower cooling costs, and mitigate the urban heat island effect. Hartzell is an ecohydrologist and studies water use by vegetation in dryland and urban ecosystems. Hartzell recently received a $193,418 grant from the National Science Foundation to study green roof vegetation and improve tools used for modeling some of the benefits of green roofs.
The ecosystem services provided by green roofs vary by vegetation choice. And the options of vegetation come with trade-offs such as water consumption and cooling effects. Local environmental conditions also affect the decision-making around vegetation choice and the resulting benefits of green roofs.
Hartzell's study aims to develop tools policy-makers and green roof designers can use to evaluate vegetation choices and the valuation of municipal green roof incentives.
CAM (crassulacean acid metabolism) plant species are a frequent choice of vegetation for green roofs because of their hardiness; they collect CO2 during the night, which helps members of the species retain water during the day. Hartzell plans to build upon recent advances in CAM modeling to accurately predict water use and ecosystem services of green roofs with various vegetation types and environmental conditions.
With funding from the NSF, the research team will construct miniaturized green roofs using Sedum plants, warm-weather grass, and cold-weather grass and measure factors including runoff and energy partitioning. The research team will then use the data to improve green roof models.
"There's been a recent increase in green roof policy guidelines, mandates, and incentives," Hartzell said. "Yet, there's still not a lot of guidance on which types of vegetation are best to use and in which climates. So, we're hoping to build a model to help us better understand these variables."
Hartzell notes that the research will also help improve our understanding of dryland ecosystems where CAM plants are commonly found by constraining expected carbon and water fluxes, particularly under a changing climate.
As green roofs become a more dominant feature of the urban landscape--and one that contributes to the resiliency of cities, research conducted by Hartzell's research team will increasingly inform policy-making and design decisions as they pertain to the ecosystem benefits of green roofs.