Project overview
Urban trees reduce the volume of stormwater runoff entering waterways by intercepting rain in their canopies. They redirecting the intercepted rain along their trunks and driplines, where the rain infiltrates into the soil. Some of that water is then taken up through the tree’s roots and transpired back into the atmosphere. However, trees have also been shown to release nitrogen, phosphorus, and carbon into the environment. Elevated concentrations of these nutrients negatively impact water quality in lakes and streams. Urban trees are therefore both stormwater sinks and nutrient pollution sources.
The research team measured elevated concentrations of nutrients in throughfall (rainwater that has passed through a tree’s canopy), as well as reductions of stormwater runoff volume, attributable to individual urban trees. These measurements allowed the team to quantify the tradeoff between nutrient release and runoff volume reduction at the scale of individual trees.
Stormwater fluxes were monitored both before and after trees were removed as part of the City of St. Paul’s Structured Ash Removal program, from 2022 to 2024. This planned tree removal project served as a response to a city-wide Emerald Ash Borer infestation, while also providing a rare opportunity to conduct a natural experiment that allowed the team to directly attribute changes in stormwater quantity and quality to individual trees.
Research questions
- How much stormwater runoff can be reduced by urban tree canopies across seasons?
- Does reduction in stormwater volumes coincide with reduction in C, N, and P fluxes?
- How do these effects vary under different rainfall intensities and across multiple years?
Research Findings
- Healthy trees with few bare branches had higher transpiration rates relative to unhealthy trees with more bare branches.
- Throughfall contained very high concentrations of nitrogen and phosphorus relative to open precipitation. For example, average soluble reactive phosphorus (SRP) concentrations in maple and ash throughfall in Highland Park were more than ten times higher than SRP concentrations in open precipitation. The amount of runoff volume reduction due to canopy interception was not enough to compensate for the large increase in nutrient concentration, thus resulting in a net nutrient flux into stormwater runoff.
- The amount of nutrient flux in throughfall varied greatly with site location, total weekly precipitation, and percent canopy interception, suggesting a wide range of responses to precipitation inputs.
- Impervious surfaces (such as sidewalks, curbs, and patios) inside of tree driplines prevent the nutrient-dense throughflow from infiltrating into the soil, where it could serve as fertilizer. Instead, impervious surfaces direct these nutrients to receiving waterways, where they act as pollutants.
Key innovations/contributions
This project produced empirical data documenting the observed amount of stormwater retained by urban trees, as well as the observed amount of nutrients that urban trees contributed to stormwater runoff.
What does this mean for Minnesota?
By providing empirical evidence of these tradeoffs, cities are now equipped with quantitative data that they can use to make management decisions regarding urban trees, weighing the potential costs of nutrient export from throughfall and litterfall against the benefits, such as runoff volume reduction, heat island mitigation, and improved mental health. These findings also indicate that cities could strategically site urban trees so that their nutrient-rich throughfall infiltrates into the soil, feeding the tree and surrounding vegetation, rather than directing it into gutters and waterways. When urban foresters base their decisions on an accurate evaluation of the tradeoffs, cities will be able to reduce nutrient pollution while preserving the ecological and social benefits of urban trees.
Reports and Presentations
- Research brief 2026 .pdf
- Project update 2024 .pdf
- Final report 2025