Urban trees provide a variety of ecosystem services to urban and suburban areas, including carbon uptake, climate amelioration, energy reduction, and stormwater management. Tree transpiration, in particular, modifies urban water budgets by providing an alternative pathway for water after rain events. The relative importance of environmental and biological controls on transpiration are poorly understood in urban areas, yet these controls are important for quantifying the ecosystem services that urban trees provide and predicting how urban ecosystems will respond to climate changes. The objectives of our study were to quantify the annual cycle of tree transpiration in an urban ecosystem and to determine how different urban tree species and plant functional types respond to environmental drivers. We continuously measured whole-tree transpiration using thermal dissipation sap flow at four urban forest stands that were broadly representative of the species composition and tree height found in an urban forest inventory of a suburban residential neighborhood of Minneapolis-Saint Paul, Minnesota. A total of 40 trees, including coniferous and deciduous, and early- and late-successional species, were sampled throughout the 2007 and 2008 growing seasons (April-November). At each site we monitored soil moisture, air temperature, and relative humidity continuously, and we measured leaf area index weekly.
Urban tree transpiration was strongly correlated with diurnal changes in vapor pressure deficit and with seasonal changes in soil moisture and leaf area index. We also observed inter-annual differences in transpiration rates due to a mid-season drought and longer growing season in 2007, compared with the cooler, wetter conditions in 2008. We found larger differences in transpiration rates between early- and late-successional species than between coniferous and deciduous species. These results were scaled up using Forest Inventory and Analysis data to estimate the relative contributions of each tree type at the scale of a 4x4 km urban landscape. These results have implications for understanding the role of trees in managing urban water budgets and predicting the impacts of climate change on urban ecosystem services.