Water sources of urban trees in the Los Angeles metropolitan area

In semi-arid urban areas, trees are likely to be irrigated, but they may also be using groundwater for transpiration. The uncertainty surrounding this issue presents challenges for management. To better understand where urban trees in Los Angeles are getting their water, this study looked at a number of species growing under different conditions (urban and natural) across the Los Angeles Basin to determine their sources of water. At each sampling site, samples of tree xylem, soil, irrigation water, and runoff were collected. Rainwater and groundwater samples were also collected. Oxygen and hydrogen isotope ratios (δ18O and δD) within the xylem water and the water samples were analyzed to distinguish the source of the water. The results indicated that the trees’ sources of water differed substantially by location and species. For example, at the LA Arboretum, groundwater use ranged from 0% for some species to 90% for others. At other sites, maximum groundwater use was 24%. At six sites, isotope ratios indicated that some species were using a third source of water that wasn’t accounted for or that processes within the stem were altering the ratios. From a management perspective, the results suggest that supplemental irrigation of trees may not always be needed, and also that groundwater uptake by trees can be significant and should be accounted for in cases where the water table is being carefully managed.