DISSERTATION RESEARCH: A Stable Isotope Analysis of Riparian Tree Water Sources Along a Gradient of Ground Water Availability

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Riparian forests that occur in the Sonoran Desert are important, yet threatened ecosystems in the southwestern United States. In the past one hundred years, impoundments and diversions of surface water and removal of ground water for human use have contributed to a decline in these ecosystems. This decline has included a loss of native species, such as cottonwood and willow trees, and the spread of exotic species, such as saltcedar. These species rely heavily on ground water as a water source, but they may use water from the surface soil if ground water is less available. Ground water availability in southwestern riparian ecosystems can vary dramatically from year to year as a result of climatic variations. The PIs observed significant declines in the health of native cottonwoods and willows, but not of the exotic saltcedar, in 1997, a dry year, when ground water was less available. In contrast, no declines in the health of any species were observed in 1998, which was a wet year. The PIs plan to use analysis of naturally occurring isotopes of hydrogen in water to determine whether native and non-native species used the same water sources, and whether water source use differed between the dry and wet years. This information will be helpful in understanding how changes in ground water availability by human manipulation and climatic variability affects desert riparian forests.