In the semi-arid southwestern U.S., urban landscapes often contain large variety of species imported from mesic environments. Exposure of these species to dry air may result in high water use and transpiration patterns that are hard to predict a priori. As the worst drought on record is ongoing in California and further decreases in water supplies are projected due to regional climate change trends, there is a critical need for a detailed understanding of water use by landscape trees in order to develop efficient water conservation strategies. We measured transpiration of 108 irrigated trees in situ in the Los Angeles metropolitan area and compared these data to published values from Salt Lake City, Utah and Duke Forest, North Carolina. We asked the questions: Are transpiration patterns of urban trees similar among semi-arid regions? Are transpiration responses to environmental factors similar or different among urban and natural environments?
Results/Conclusions
Despite climatic and environmental differences among study regions, similar species in different habitats had similar responses of normalized transpiration to atmospheric vapor pressure deficit (VPD) within overlapping ranges of VPD. Moreover, an empirical model based on our in situ measurements of urban transpiration in Los Angeles was in good agreement with transpiration of urban trees in Salt Lake City, riparian trees in Salt Lake City, and trees in Duke Forest averaged over the duration of the growing season. We conclude that tree transpiration is similarly controlled by environmental factors in both mesic and semi-arid regions, despite vastly different ranges of VPD. Previous studies have shown unique patterns of transpiration in irrigated trees in urban areas, but our findings indicate that these patterns are predictable based on species-specific stomatal responses to VPD and solar radiation that do not differ substantially in mesic vs. semi-arid or in natural vs. urban forests. These findings are promising for improving regional hydrologic models in disparate regions and for developing water wise strategies for a variety of tree species grown in urban landscapes.