In semi-arid cities, urban trees are often irrigated, but may also utilize natural water sources such as groundwater. Consequently, the sources of water for urban tree transpiration may be uncertain, complicating efforts to efficiently manage water resources. We used a novel approach based on stable isotopes to determine tree water sources in the Los Angeles basin, where we hypothesized that trees would rely on irrigation water in the soil rather than develop deep roots to tap into groundwater. We evaluated the oxygen (δ18O) and hydrogen (δD) isotope ratios of xylem water, irrigation water, soil water, and groundwater in a study of temporal patterns in water sources at two urban sites, and a study of spatial patterns at nine urban sites and one “natural” riparian forest. Contrary to our hypothesis, we found that despite frequent irrigation, some trees tap into groundwater, although in most species this was a small water source. Some trees appeared to be using very shallow soil water at <30 cm depth, suggesting that these mature urban trees were quite shallowly rooted. In the natural site, trees appeared to be using urban runoff in addition to shallow soil water. We were able to identify tree uptake of precipitation at only 3 sites. The results show that some irrigated trees utilize groundwater and do not rely solely on irrigation water, which may make them able to withstand drought and/or water conservation measures. However, some irrigated trees may develop very shallow root systems, which may make them more susceptible.
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We thank UC Irvine Facilities Management, the City of Los Angeles Urban Forestry Division, the Los Angeles Zoo and Botanical Gardens, The Los Angeles County Arboretum and Botanic Garden, the Los Angeles Police Academy, and the Starr Ranch Audubon Sanctuary for access to their properties. Ibrahima Diallo, Gabriel Giannini, and Eric Sun provided valuable assistance with field sampling and laboratory analyses. We thank Dr. Theodore von Bitner of the Orange County Public Works Department and Vivian Marquez, Jonathan Ball, and Bryan Truong of the Pollution Assessment Section of the City of Los Angeles, Watershed Protection Division, Bureau of Sanitation for help with groundwater sampling. This research was funded by the U.S. National Science Foundation grant 0624342, EPA Star grant RD-83336401-0, and a U.S. National Science Foundation Graduate Research Fellowship.
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Bijoor, N.S., McCarthy, H.R., Zhang, D. et al. Water sources of urban trees in the Los Angeles metropolitan area. Urban Ecosyst 15, 195–214 (2012). https://doi.org/10.1007/s11252-011-0196-1
- Tree water sources
- Urban forest
- Stable isotopes