Water sources of urban trees in the Los Angeles metropolitan area

Abstract

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 (δ 18 O) 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.

In situ monitoring of H and O stable isotopes in soil water reveals ecohydrologic dynamics in managed soil systems

“…In a study of tree water use in an urban setting, Bijoor, McCarthy, Zhang, and Pataki (2012) found a similar pattern of xylem water H and O isotope values that lie below and to the right of the LMWL on a dual isotope plot and interpreted those results to indicate that the trees were using shallow soil water (<30 cm soil depth). In a study of tree water use in an urban setting, Bijoor, McCarthy, Zhang, and Pataki (2012) found a similar pattern of xylem water H and O isotope values that lie below and to the right of the LMWL on a dual isotope plot and interpreted those results to indicate that the trees were using shallow soil water (<30 cm soil depth).…”

Prevalence and magnitude of groundwater use by vegetation: a global stable isotope meta-analysis

“…This fundamental understanding of water uptake and transport from roots to shoots underlies the utility of stable isotopes in plant water uptake investigations17. While many site-based studies have now been completed18192021222324252627282930313233343536373839404142434445464748495051525354555657585960616263646566676869707172737475767778798081828384858687888990919293949596979899100101102103104105106107108109110111112113114…”

Urban Heat Stress Vulnerability in the U.S. Southwest: The Role of Sociotechnical Systems

“…Trees and watered vegetation (when it exists), can offer respite from the warmest days [79] but southwest cities have less tree canopy than cities in more temperate climates. Temperate cites tend to be located in places that were forested while tree canopy in southwest cities is a result of afforestation efforts and must be balanced with water resources [80][81][82]. Yet, urban shading is a component of cooling the urban atmosphere and an aspect of infrastructure.…”