Impoundments create unnatural shorelines that differ from lake shorelines in patterns of water-level fluctuations, flow, sediment transport, and shoreline vegetation dynamics. Shoreline plant communities of impoundments in the American Southwest often become dominated by mature, senescent Populus and Salix, with few if any seedlings. The failure of native plant community replacement is exacerbated by the fact that Tamarix, a prolific invader, is abundant on regulated rivers and occupies extensive areas along the shores of impoundments. Efforts to replant natives within the drawdown zone surrounding Lake Mohave, a lower Colorado River impoundment bordering Nevada and Arizona, have not been successful. A greenhouse experiment was designed to examine the responses of cuttings of a native species, Salix gooddingii (Goodding willow), and the invasive species, Tamarix ramosissima (salt cedar), to different hydroperiods comparable to those influencing Lake Mohave riparian plant communities. Higher survival and greater biomass under saturated but not flooded soil conditions demonstrated that both Salix and Tamarix cuttings can prosper in soils within the exposed drawdown zone, provided that the shoots are not submerged. However, greater biomass of Tamarix under conditions favorable to the native Salix also indicates that Tamarix colonization will have to be controlled. This research substantiates that the physiological tolerances of native and invasive riparian plant species and the prevailing hydrodynamics of a waterway must be considered when attempting to restore native vegetation.
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Vol. 22 • No. 4