1NATURAL RECOVERY OF NATIVE RIPARIAN TREE SPECIES TO EXOTIC-DOMINATED SITES: INFLUENCE OF PHYSICAL PROCESSES AND BIOTIC INTERACTIONS

Groundwater and surface flow depletions are altering riparian ecosystems throughout the southwestern United States. Due in large part to hydrologic alterations and land use changes, forests of the pioneer trees Populus fremontii (Fremont cottonwood) and Salix gooddingii (Goodding willow) have declined. In many areas these forests have been replaced by shrublands and woodlands of Tamarix ramosissima (salt cedar). Previous work has focused on the physiological responses of these riparian plant species to decreases in water availability, but little attention has been given to shifts in community and population structure (size class diversity, canopy cover, basal area, vegetation volume, and stem density) in response to declines in surface flow duration and groundwater levels. This study identified hydrologic thresholds above which P. fremontii-S. gooddingii maintain tall dense stands with diverse age classes, and above which they are more abundant than T. ramosissima stands along the San Pedro River in Arizona. Populus fremontii and S. gooddingii were dominant at sites where average maximum depth to groundwater was less than 2.9 meters (m), annual groundwater fluctuation was less than 0.6 m, and surface flow was present more than 73 % of the month