Floodplain vegetation following over 80 years of intensive land use and de-watering :\ud Lower Owens River, California.

Graduation date: 2008Riparian areas in the arid western United States are critical ecosystems that have been\ud severely degraded by a variety of land and water uses over the last 100 years. In this\ud study, the composition and structure of floodplain vegetation along the Lower Owens\ud River in eastern California was quantitatively described following over 80 years of dewatering,\ud grazing, groundwater pumping, and other land uses. The Lower Owens River is\ud a heavily impacted and invaded riparian system which has suffered long-term hydrologic\ud alteration and degrading land use. This study is part of the Lower Owens River Project,\ud an effort to restore ecosystem function to this riverine landscape.\ud Using a stratified design, five 2km long study plots were established in five river reaches\ud within the 86 km study area. Within these plots, dominant species were ranked by cover\ud in six structural classes along 105 line-intercept transects and species canopy cover and\ud ground cover was estimated at 525 2m x 2m sub-plots in 2001 and 2002 to determine\ud vegetation type composition and condition. Twenty–two vegetation types were delineated\ud by cluster analysis and Indicator Species Analysis (ISA). Based on species composition\ud and dominance, cluster analysis and Non-metric Multi-dimensional Scaling (NMS) were\ud employed to examine relationships between vegetation types and environmental\ud parameters. Vegetation composition and species diversity indices were strongly\ud correlated with geographic and hydrologic variables. A weighted wetland indicator index\ud elucidated a shift toward more xeric species and communities in the completely\ud dewatered reaches, as well as a negative correlation with diversity measures. Completely\ud dewatered reaches were dominated by communities that have shifted from native to\ud exotic vegetation types. Five of the 22 distinct vegetation types delineated were\ud dominated by exotic species, mainly salt-cedar (Tamarix spp.) and Russian thistle\ud (Salsola tragus). These communities covered approximately 24% of the study area, and\ud dominated the most severely dewatered reach. Conversely, native communities remained\ud dominant in reaches where surface water was present. Native vegetation types covered\ud 96% of these reaches, an increase of 64% (p=0 from a randomization test) over the\ud severely dewatered reach. The native willow (e.g. Salix goddinggii and Salix exigua) and\ud wet meadow vegetation types exhibited the highest species diversity, and therefore\ud represent target vegetation types for restoration. The results of this study elucidate the\ud changes in vegetation type cover and composition following dewatering and degrading\ud land use in an arid western riparian system