Improved understanding of water use and drought tolerance in urban trees is needed to better match species to planting environment and in understanding potential climate change effects on urban trees. The goal of this study was to quantify whole-tree water use from thermal dissipation measurements of sap flow and drought tolerance in eight mature deciduous tree species commonly planted in southern urban landscapes. These species were Quercus acutissima, Quercus michauxii, Quercus alba, Quercus nuttallii, Quercus rubra, Ulmus parvifolia, Fraxinus pennsylvanica, and Cercis canadensis. Specific objectives were to: 1) quantify water use patterns in irrigated and non-irrigated trees, and 2) compare drought tolerance defined by whole-tree hydraulic conductance, leaf water potential, and leaf-level physiology among species. Transpiration and leaf physiology were measured in irrigated and non-irrigated 12-year-old trees over a growing season.
Average daily sap flow ranged from 0.32 L cm-1 stem diameter day-1 in Cercis canadensis to 2.0 L cm-1 day-1 in Ulmus parvifolia. Ulmus parvifolia was a consistently high water user; whereas Quercus rubra and Cercis canadensis were low water users and the other species were intermediate in water use. Species differences in water use were unaffected by irrigation despite lower predawn leaf water potential with drought. Ulmus parvifolia, Quercus acutissima and Quercus nuttallii appear to be drought avoiders as leaf water potential and whole-tree transpiration were high throughout the growing season. Fraxinus pennsylvanica, a popular urban tree, tolerated drought, as shown by high whole-tree conductance and sustained transpiration rates with decreasing water potential. The drought avoiders may maintain plant water potential and avoid drought stress as a result of an extensive rooting system, stem capacitance, higher sapwood to diameter ratios and low resistance to water transport. However, Cercis canadensis and Ulmus parvifolia has similar sapwood to stem area ratios. The ability to maintain carbon uptake with high evaporative demand has been suggested as an ideotype in intraspecific selection of species for urban sites. These results suggest that Ulmus parvifolia, Quercus acutissima and Quercus nuttallii fit this ideotype.