Managing Stormwater with Urban Forests: Transpiration Rates and Whole-Tree Water Use by Deciduous Species Varies across Management Contexts

Urban trees can reduce stormwater runoff, mitigate flood risk, increase infiltration and water storage capacity in the soil, reduce nutrient loading, and improve water quality in developed areas. In this study we investigate the impacts of management contexts and different deciduous tree species on transpiration rates and whole-tree transpiration, as well as the relationships between environmental drivers and tree transpiration. We monitored sap flux rates of red maple, tulip poplar and sweetgum trees from June 2018 to November 2019 in urban forest patches in Baltimore, MD, and a cluster of trees over turf grass and single trees over turf grass in Montgomery County, MD. We found significantly lower sap flux density in closed-canopy trees, compared to solitary or small clusters of trees. No significant differences were observed between whole-tree transpiration across tree species. We estimated that trees transpired between 17.4 to 60.7% of rainfall volume that fell on the crown of individual trees during the study period. This study will enhance the ability of urban forest managers and policymakers to make decisions about stormwater management, as well as provide insight about how species and management context interact with climate to affect resilience to current and future climate stress. Specifically, we have developed whole-tree water use estimates for hypothetical trees that stormwater managers can use with only DBH, species, and management context. For instance, this project can aid the development and refining of stormwater crediting programs as urban trees can be more accurately incorporated into planning efforts. Additionally, insights from this study about potential differences in transpirational behavior can guide future scaling up and modeling of stand transpiration as well as regional water flux models.