Plant water-use efficiency as a metric of urban ecosystem services

Trees in urban ecosystems are valued for shade and cooling effects, reduction of CO2 emissions and pollution, and aesthetics, among other benefits. However, in arid and semiarid regions, urban trees must be maintained through supplemental irrigation. In these regions it is desirable to identify tree species that are especially efficient in the balance between water loss and carbon uptake. We used a common-garden approach to compare water-use efficiency (WUE) at leaf and tree scales for commonly planted, nonnative tree species in the Los Angeles Basin (California, USA), in order to evaluate WUE as a metric of the trade-off between water use and growth in urban trees. Leaf-level gas exchange, sap flux density, leaf delta C-13, and stem growth measurements were conducted on eight species within the Los Angeles County Arboretum: Brachychiton discolor, B. populneus, Eucalyptus grandis, Ficus microcarpa, Jacaranda chelonia, Gleditsia triacanthos, Lagerstroemia indica, and Koelreuteria paniculata. We found species with high instantaneous WUE also had the highest tree-level seasonal WUE (stem basal-area increment (BAI)/ total transpiration). High tree-level WUE resulted from low water use in B. discolor, B. populneus, and E. grandis. In contrast, high basal-area growth explained moderately high WUE in F. microcarpa. Notably, high WUE was not associated with low BAI. At a monthly time scale, nearly all species showed the highest WUE during late spring/early summer, when the majority of basal-area growth occurred. Although leaf-and tree-level WUE were reasonably well correlated, leaf delta C-13 was not significantly related to leaf-or tree-level WUE. Overall, the most water-efficient species were evergreen, or from regions that experience high vapor-pressure deficit (VPD). These results suggest that whole-tree WUE is a useful measure of the balance between some critical costs and benefits of irrigated urban trees and may be helpful in determining which trees should be planted to maximize growth while conserving water. Although measuring whole-tree WUE directly provides the most complete understanding of urban tree costs and benefits, this study suggests that leaf-level instantaneous measurements of WUE and knowledge of species native climates may be reasonable proxies.