Causes and management of exotic riparian plant invasion in Canyon de Chelly National Monument, Arizona

The ecological, economic and social impacts of invasive plant species on native plant communities have stimulated broad concern among researchers, land managers and the general public. Riparian areas are of particular concern because they are critical to regional biodiversity despite covering a small percentage of the landscape. In the southwestern United States (U.S.), the most dominant riparian plant invaders are the woody species tamarisk ( Tamarix ramosissima Ledebour, T. chinensis Loureiro, and their hybrids) and Russian olive (Elaeagnus angustifolia L.). Tamarisk and Russian olive have invaded riparian habitats throughout Canyon de Chelly National Monument in northeastern Arizona. The goals of my research were to: (1) describe the history and mechanisms of exotic plant invasion into Canyon de Chelly, (2) understand the establishment niche space requirements of tamarisk, Russian olive and native cottonwood in terms of light and water and determine if tamarisk and cottonwood are facilitating the invasion of Russian olive, and (3) describe response of the riparian ecosystem to exotic plant removal and determine the effectiveness of two different removal strategies. My results from analyzing the history of invasion showed that although plantings and river regulation by dams probably played a role in tamarisk and Russian olive invasion into Canyon de Chelly, these species required hydroclimatic drivers and stream bed adjustments for wide-spread establishment. Controlled experiments and field surveys in my second study demonstrated that Russian olive is exploiting empty niches along wide gradients of water and light availability in southwestern riparian ecosystems. However, Russian olive invasion does appear to be limited by seed dispersal. Finally, I found that both cut-stump and whole plant removals similarly reduced exotic species cover and increased native species cover after two years. Both removal methods also reduced aerial seed rain inputs of tamarisk seeds, cut-stump removals increased available nitrogen near dead Russian olive boles within two years of removal, and both treatments seem to have no effect on ground water levels. This research can guide the management of riparian plant communities in Canyon de Chelly, across the southwestern U.S., and inform our understanding of exotic plant invasions.