Do irrigated trees transpire irrigation water in the Salt Lake City metropolitan area?

Background/Question/Methods

Plant transpiration is a critical process in urban ecohydrology and has great potential to mitigate the urban heat island effect. The urban forest in western U.S. semi-arid cities is composed largely of non-native species sustained by irrigation. Given that in these areas the major end use of water is irrigation, the efficiency of irrigation practices is an essential factor in future water conservation measures. We investigated whether urban trees are using irrigation as the primary source of transpiration and if this depends on groundwater depth or on particular characteristics of the species. We chose five irrigated parks in the Salt Lake City Metropolitan area with differing water table depths (0–3m or >30m). For deep groundwater parks, we expected trees to use mostly irrigation water due to high costs of growing a vertically extensive root system. We collected groundwater, tap water (assuming it has the same composition as irrigation water) and tree stems of six commonly grown species. To identify the water sources of the trees, we compared the composition of stable isotopes of oxygen (δ¹⁸O) and hydrogen (δ²H) of the stem water and sources. 

Results/Conclusions

The results showed that groundwater differs in isotopic composition from tap water for all parks (p<0.01). Both sources fell close to the local meteoric water line indicating low evaporation. The water table depth was not a determinant of the isotopic composition of stem water (p=0.056). There were no significant differences in stem water isotopic composition among species except for Picea pungens (p<0.05). The isotopic analysis indicated that all species appeared to be capable of reaching groundwater in at least one locality indicating their ability to grow an extensive vertical root system. The results indicate that irrigated urban trees in the Salt Lake City Metropolitan area may not rely heavily on irrigation water in some locations. Therefore, some urban trees can withstand reductions in irrigation rates. Future studies will explore other possible water sources such as runoff and precipitation as well as seasonal patterns in the use of irrigation water to evaluate temporal and spatial shifts in water sources. This study based on an ecological approach will contribute to understanding the biological controls on the water cycle in irrigated cities and will help identify tradeoffs in urban ecosystem services versus maintenance costs.