PATTERNS IN SHORELINE VEGETATION AND SOILS AROUND LAKE MOHAVE, NEVADA AND ARIZONA: IMPLICATIONS FOR MANAGEMENT

Lake Mohave, on the lower Colorado River in Nevada and Arizona, was created by the construction of Davis Dam for power generation, flood control, and water supply. Management has led to the periodic lowering of the water level of the reservoir (drawdown), such that it reveals a gradient of zones around the margins of the reservoir that range from frequently inundated to frequently dry. The initial filling of Lake Mohave flooded the preexisting native riparian woodlands of Populus-Salix (cottonwood-willow), creating a new shoreline and plant community. We analyzed the spatial distribution of the plant species that dominate the plant community (i.e., native Salix g,gooddingii C. R. Ball [Goodding's willow] and nonindigenous Tamarix ramosissima Ledeb. [saltcedar]) and the soil components to discern patterns. Data analyses and modeling indicate that there are 3 emergent patterns in the distribution and composition of vegetation and soils. First, even though both S. gooddingii and T ramosissima were present in the inundated zones, there were more mature S. gooddingii individuals in the frequently inundated reaches, while T ramosissima presence and cover increased with distance from the water's edge. Salix gooddingii seedlings were not observed, but T ramosissima seedlings were present in all zones. The only regeneration of S. gooddingii was vegetative. Naturally occurring Populus fremontii S. Watson (Fremont cottonwood) was completely absent in the drawdown and upland plant communities. Second, soil salinity and pH values range from 49.4 to 0 dS . m(-1) and 6.4 to 9.4, respectively, and varied significantly with landform type and geographic location along the reservoir Patterns in soil chemistry may be related to shore geomorphology that either shelters or exposes soils to wave action, which mechanically agitates, aerates, and flushes soils. Presence of Salix gooddingii in the frequently inundated zones and the co-occurrence of T ramosissima and relatively high soil salinity concentration reflect patterns among plant flood tolerance and soil responses to periodic inundation. While reasons for the absence of P fremontii are unknown, the absence of S. gooddingii seedlings may be related to the fact that seed release coincides with the period when the reservoir is at its highest, thereby limiting recruitment. Third, the only regeneration of S. gooddingii appeared to have occurred following herbivory (Castor canadensis Kuhl [North American beaver]) and wind damage. We conclude with suggestions for the conservation of novel riparian ecosystems as surrogates for lost native ecosystems. These suggestions include manipulating reservoir water levels to simulate natural fluvial processes so that nonnative plant establishment is inhibited, excessive soil salts are flushed from the system, and native transplants can be established.