Invasive species are increasingly problematic world-wide. Scientists working to understand why invasive species are successful must first understand the processes of invasion. In the southwestern United States (U.S.) the two most tenacious invaders of riparian habitats are the exotic woody plant species tamarisk (Tamarix ramosissima, T. chinensis, and their hybrids) and Russian olive (Eleagnus angustifolia). These plants were introduced by humans throughout the southwest around 1900, and may interact with other human land management activities such as river regulation. Tamarisk and Russian olive were planted during the 1930s in Canyon de Chelly National Monument, Arizona, to improve farming and protect archeological sites. In 1963, two dams were installed in the headwaters of the Canyon de Chelly watershed. We hypothesized that one of three factors triggered exotic plant invasion in Canyon de Chelly: the historic plantings, river regulation by dams, or twentieth century climate condition. We precision-aged 180 tamarisk and Russian olive samples from the study canyons, and used tree ring analysis to determine the year of establishment, and the elevation of the germination point relative to the channel.
Results/Conclusions

Our results indicate that exotic plants invaded a bare, sandy channel, establishing in habitat that native plants were not occupying. The majority of tamarisk and Russian olives established in the mid to late 1980s, long after the original plantings and dam construction. These results suggest invasion of exotic plants in Canyon de Chelly is more closely tied to climatic variables and available seedling habitat, rather than river regulation or purposeful plantings. We also found that tamarisk establishment has largely ended in the upper reaches of the canyon, whereas Russian olive establishment is on-going. Russian olive can successfully establish in the shaded habitats well above the water table where tamarisk cannot. It is anticipated that Russian olive will continue to increase in dominance in this southwestern habitat while tamarisk decreases.