PLANT COMMUNITY RESPONSE TO TAMARISK INVASION AND HYDROLOGIC REGIME IN THE CATARACT CANYON, CANYONLANDS NATIONAL PARK: A PRELIMINARY INVESTIGATION UNR-NPS Task Agreement # J8R07070014 Great Basin Cooperative Ecosystems Study Unit Cooperative Agreement # H

In the southwestern US, the composition and abundance of riparian plants has changed as a result of flow regime alteration and the invasion of Tamarix. We investigated trends in vegetation dynamics along the Green and Colorado Rivers in Canyonlands National Park (CNP) from 1976 to 2008 through historical aerial photo analyses. We also explored the relationships between understory and overstory species in riparian habitat of CNP. The effects of river regulation on seedling establishment patterns were assessed in slightly regulated, moderately regulated, and extremely regulated river sections in the southwestern US. Tamarix dominated the overstory vegetation in all river sections in CNP but was less dominant along the Colorado (73 % vs. 83%-86 % for the Green and Cataract sections). Overstory-understory relationships were generally quite weak, although species commonly found with Tamarix included Bromus tectorum, Distichlis spicata, and Sporobolus spp. Seedlings of Tamarix and Salix exigua were abundant, with the latter being far more abundant than below the Glen Canyon Dam in Grand Canyon. However, seedlings of Populus fremontii and Salix gooddingii were rare. Apparent differences in the affinities of Tamarix and Salix exigua for certain soil characteristics including pH, conductivity, and relative concentration of ammoniu