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 semi-arid regions, urban trees must be maintained through supplemental irrigation. In these regions it is useful to identify tree species which are especially efficient in the balance between water loss and carbon uptake. Thus, the objective of this study was to use a common garden approach to compare water use efficiency (WUE) at leaf and tree scales for commonly planted, non-native tree species in the Los Angeles Basin. Leaf level gas exchange, sap flux density, leaf δ¹³C and periodic stem growth measurements were conducted on eight species within the Los Angeles County Arboretum: Brachychiton discolor, Brachychiton populneus, Eucalyptus grandis, Ficus microcarpa, Jacaranda chelonia,Gleditsia triacanthos, Lagerstroemia indica, and Koelreuteria paniculata. We wished to answer the questions: Which species are most efficient in their water use? How well are water use and carbon gain correlated at the leaf and tree scales (i.e. are trends across the species similar for both scales)? And, how well do leaf-level gas exchange and δ¹³C based estimates of WUE correspond in this system?

Results/Conclusions

At the leaf scale, instantaneous WUE based on leaf-level gas exchange was highest in B. discolor, E. grandis and F. microcarpa. Species with high instantaneous WUE also had the highest tree level, seasonal WUE (stem basal area increment / total transpiration), although species differences in WUE were much more pronounced at the larger scale. High tree-level WUE resulted from low water use in B. discolor, B. populneus and E. grandis. In contrast, high (and sustained) growth in F. microcarpa explained high WUE in this species At a monthly time scale, nearly all species showed the highest WUE during late spring/early summer, when the majority of stem growth occurred. Leaf δ¹³C was not significantly related to leaf or tree level WUE. Generally, the most water efficient species were evergreen, and from regions of the world which experience high vapor pressure deficits. Notably, several species planted for their attractive flowers (e.g. J. chelonia and L. indica) had very low water use efficiency. These results can contribute to the currently small pool of data on urban tree water use and ecophysiology.