The role of planted forests in urban water budgets (Invited)

In arid regions which are not naturally forested, urban trees are sustained through the redistribution of water resources as irrigation. Assessments of outdoor water use in Southwestern US cities have shown that not only is 30-75% of residential water use expended on outdoor landscapes, but that irrigation is frequently in excess of estimated plant demand. Thus, there is a need to understand the factors which influence the magnitude and variability of water use of urban trees. A complicating factor in assessing urban tree water use is the widely recognized heterogeneity of urban environments. Human choices and decision-making result in a landscape with significant variability in water and nutrient inputs, microclimate, biotic inputs and vegetation composition. In order to quantify urban tree water use and explain variation in water use resulting from variability in resource availability and species composition, we have conducted a combination of sapflux, growth and isotopic studies on more than 35 common (primarily non-native) tree species in the Los Angeles basin. The objective of these studies was to determine how much variability in water use and water use efficiency exists within and between commonly planted urban tree species, and what factors explain or can be used to predict this variability. Through these studies we have found considerable differences (up to two fold) in tree transpiration within a given species, attributable to differences in water and nutrient availability and tree planting density. Additionally, we have found substantial variation in the water use of different species: at typical urban planting densities, peak transpiration rates can be more than ten times greater for high transpiring trees than low transpiring trees. Finally, we found whole tree water use efficiency to vary across species by a factor of up to a hundred, explained to a large degree by the climate conditions (especially vapor pressure deficit) in the native ranges of these non-native trees. On the scale of the entire city of Los Angeles, we estimate that the urban forest could use as much as 50% of the total municipal water use. Overall, we have found that urban trees can use substantial quantities of water, and that species choice matters greatly in determining urban landscape water use.