Evaluating the effectiveness of urban trees to mitigate storm water runoff via transpiration and stemflow

Many cities in the Eastern United States are working to increase urban tree cover due to the hydrological services that trees provide, including the interception, storage and transpiration of water that would otherwise enter sewer systems. Despite the understanding that trees benefit urban ecosystems, there have been few studies that address the underlying physiology of different urban trees with regard to their capacity to take up water, particularly following rain events. We monitored the sap flow of nine species of trees in urban parks and linked sap flow to local microclimate. In addition, we measured throughfall, stemflow and crown architecture. Interspecific variation in sap flow was significant as were differences in time lags (i.e. difference in time between the increase of solar radiation versus sap flow) across species of large but not small trees. Interestingly the most important microclimatic drivers of sap flow were different in large versus small trees. Lastly, we found that trees with a large branch angle routed more rainwater to stemflow. In this study we found strong evidence that variation in urban tree physiology can impact important hydrological services that will influence the effectiveness of different trees as tools to manage stormwater runoff.