Urban trees influence stormwater quantity and quality from site to watershed scales

Trees are found ubiquitously in urban environments and are appreciated for a range of ecosystem services. However, their ability to reduce stormwater runoff volumes – and any tradeoffs involved with nutrients and other pollutants typically carried by stormwater runoff – are largely overlooked in stormwater management practices due to lack of robust data. We undertook a three-year project to monitor tree-scale water quantity and quality fluxes and examine how these local measurements and patterns related to the watershed observations across the city of Saint Paul, Minnesota. In this conference paper, we report on our multi-year effort to quantify the effects of trees on the urban hydrologic cycle, which included measuring the stormwater interception capacity of urban trees and their contributions to coarse organic matter, nitrogen, and phosphorus fluxes, using a series of watersheds in the Saint Paul, Minnesota, USA. At the tree scale, we quantified patterns in canopy throughfall amount, transpiration, and nutrient fluxes in canopy throughfall, which relates directly to stormwater runoff generated under deciduous trees. Under most species at most sites, canopy throughfall was statistically lower than open precipitation. Transpiration rates, determined by the sapflux method, differed across individual trees, with tree health and canopy defoliation explaining some of the differences between individual trees. Canopy interception also altered throughfall nutrient concentrations and fluxes relative to open precipitation. In spring and summer seasons between 2023-2024 mean soluble reactive phosphorus (SRP) and total organic carbon (TOC) fluxes were significantly higher under ash and maple trees than in open precipitation despite lower overall water fluxes under the canopy. With this process, deciduous trees potentially contribute increased phosphorus fluxes to stormwater while leaves remain on the tree canopy. Beyond the individual tree scale, stormwater fluxes, as monitored at the watershed level, showed variation with landcover. The presence of deciduous street tree canopy in watersheds corresponded to patterns in nutrient concentrations in stormwater at the storm-event and seasonal time scales.