Expansion of tree cover is a major management goal in cities because of the substantial benefits provided to people, and potentially to water quality through reduction of stormwater volume by interception. However, few studies have addressed the full range of potential impacts of trees on urban runoff, which includes deposition of nutrient-rich leaf litter onto streets connected to storm drains. We analyzed the influence of trees on stormwater nitrogen and phosphorus export across 19 urban watersheds in Minneapolis-St. Paul, MN, U.S.A., and at the scale of individual streets within one residential watershed. Stormwater nutrient concentrations were highly variable across watersheds and strongly related to tree canopy over streets, especially for phosphorus. Stormwater nutrient loads were primarily related to road density, the dominant control over runoff volume. Street canopy exerted opposing effects on loading, where elevated nutrient concentrations from trees near roads outweighed the weak influence of trees on runoff reduction. These results demonstrate that vegetation near streets contributes substantially to stormwater nutrient pollution, and therefore to eutrophication of urban surface waters. Urban landscape design and management that account for trees as nutrient pollution sources could improve water quality outcomes, while allowing cities to enjoy the myriad benefits of urban forests.
Bibliographical noteFunding Information:
This study could not have been possible without the sustained efforts of committed people, agencies and residents of the cities involved in this study. We especially acknowledge Joe Knight (UMN), Britta Belden and Bob Fossum (CRWD), Mike Perniel (MPRB), Stephanie Johnson and Jen Keville (MWMO), John Loomis (SWWD), and Erik Anderson (WCD) for assistance in data acquisition and for their knowledge of the data sets and watersheds included in this study. We acknowledge Michelle Rorer and Sandra Brovold for analyzing water samples at UMN. We gratefully acknowledge financial support from CRWD, SWWD, the University of Minnesota Water Resources Center (Project ID: 2012MN314B), and the University of Minnesota Institute on the Environment (Project IDs: DG-0008-11 and DG-0007-14).