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

索诺兰沙漠中的河岸森林是美国西南部重要但受威胁的生态系统。 在过去的一百年里，蓄水和地表水的分流以及地下水的抽取供人类使用，导致了这些生态系统的衰退。 这种下降包括本地物种的损失，如棉白杨和杨柳，以及外来物种的传播，如盐雪松。 这些物种严重依赖地下水作为水源，但如果地下水较少，它们可能会使用表层土壤的水。由于气候变化，西南部河岸生态系统的地下水可用性每年都有很大变化。 在1997年，由于干旱，地下水供应不足，PI观察到本地棉白杨和柳树的健康状况显著下降，但外来的盐雪松的健康状况没有下降。 相反，1998年是多雨的一年，没有观察到任何物种的健康下降。 研究人员计划利用对水中天然存在的氢同位素的分析来确定本地和非本地物种是否使用相同的水源，以及干旱和潮湿年份之间的水源使用是否不同。 这一信息将有助于了解地下水供应的变化，人类的操纵和气候变异性如何影响沙漠河岸森林。

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.