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