Soil-plant relations along a soil-water gradient in great basin riparian meadows

Throughout the western states, riparian ecosystems have been affected by water diversions or spring and seep developments that decrease the quantity of instream flows and result in lowered water-tables. Water extraction is especially damaging in arid and semi-arid regions where the presence of instream and ground-water flows are crucial to riparian vegetation. We examined the temporal and spatial relationships between hydrologic gradients, vegetation, and soils in two central Nevada riparian meadows in order to identify plant species and environmental variables that can serve as indicators of water-table status. Species frequency and aerial cover, ground-cover composition, depth to water-table, and soil morphological and physical properties were measured along hydrologic gradients within two riparian meadow complexes. TWINSPAN, cluster, and multivariate discriminant analyses classified the vegetation into four ecosystem types. These occurred along the hydrologic gradient and included, from wettest to driest, wet meadow, mesic meadow, dry meadow, and basin big sagebrush meadow types. Canonical correspondence analysis (CCA) indicated that the variables most strongly related to plant species frequency within both meadows were those associated with depth to water-table. Integrative variables, including the number of days that depth to watertable was less than 30 cm and less than 70 cm, and degree-days of anaerobiosis were most closely related to the wet and mesic meadow vegetation types. The range in depth to water-table, elevation, and aerial cover of gravel and litter were all related to the dry and sagebrush meadow vegetation types. Indicator species associated with particular water-table regimes were identified for each vegetation type based on two-way ordered tables. Carer nebrascensis, an obligate riparian species, occurred at water-tables of 0-30 cm below the surface and was the most reliable indicator of shallow water-tables. Large temporal and spatial variability in water-table depths for the mesic and dry meadow types suggests that species associated with these types could be used only to indicate broad ranges in water-table depth. Integrative environmental variables that incorporated the temporal variation in water-tables (i.e., days that depth to water-table was less than 30 and 70 cm; degree days of anaerobiosis; range in water-table depth during the growing season) demonstrated closer relationships to the vegetation types than water-table alone. They were also more sensitive to the spatial and temporal differences in water-tables than individual plant species or vegetation types. Environmental and integrative environmental variables may respond more quickly to changes in local hydrology than plant species and are possibly more sensitive indicators of both current water-table status and potential vegetation.