Stormwater reduction and pollutant sourcing from urban trees

This proposed research aims to provide empirical data to support the use and management of urban trees as a stormwater control measure. Trees are ubiquitous in urban environments and are appreciated for a range of ecosystem services. However, their ability to reduce stormwater runoff volumes – and the nutrients and pollutants typically carried by stormwater runoff – are largely overlooked in stormwater management practices due to lack of robust data. The scant amount of data that do exist show large uncertainties due to variations in climate setting, species, and rainfall intensities. Here, we propose to fill these data gaps by measuring the water and nutrient balance associated with urban trees, including their stormwater interception capacity and contributions to coarse organic matter, nitrogen, and phosphorus fluxes, using a range of watersheds found in the Capitol Region Watershed District as case studies. We do so by leveraging the Structured Ash Removal program from the City of St. Paul.

This program provides us a rare opportunity to anticipate the future removal of ash trees, enabling us to set up our work as a natural watershed experiment to evaluate the effect of ash trees on stormwater before and after removal. Our results will provide stormwater practitioners with estimates of stormwater reduction capacity of urban trees that are species-specific and locally relevant to Minnesota cities. They will also contribute to the increased recognition of urban trees as a nature-based solution for stormwater control, to be adopted in the portfolio of green infrastructure practices.

Projected outcomes

  • Development of cost-effective environmental sensors for measuring urban trees’ impact on stormwater and microclimate, to be deployed in multiple sites across St. Paul.
  • Collection of previously unavailable data on urban canopy effects on stormwater and microclimate. Measurements of transpiration, soil moisture, throughfall, infiltration, air temperature and relative humidity, as well as throughfall chemistry on C, N, P, will be used to construct water and nutrient flux budgets for each site, across different species, at different rainfall intensities, and across a growing season. These data will be published in public databases to facilitate future use by stormwater practitioners.
  • Increased communication and information sharing between researchers, managers, and practitioners toward more effective management of urban tree canopies (via targeted presentations in relevant communities, e.g., Minnesota Water Resources Conference, Minnesota Stormwater Seminar Series, outreach to watersheds and parks districts, as well as through scientific publications in peer reviewed journals). 
  • Student training: 1 PhD and 2 MS students through programs at UMN are involved in this work, which will contribute to their theses.