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

Throughout the western states, riparian ecosystems have been affected by wa ter diversions or spring and seep developments that decrease the quantity o f instream flows and result in lowered water-tables. Water extraction is es pecially damaging in arid and semi-arid regions where the presence of instr eam and ground-water flows are crucial to riparian vegetation. We examined the temporal and spatial relationships between hydrologic gradients, vegeta tion, 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 compos ition, depth to water-table, and soil morphological and physical properties were measured along hydrologic gradients within two riparian meadow comple xes. TWINSPAN, cluster, and multivariate discriminant analyses classified t he vegetation into four ecosystem types. These occurred along the hydrologi c 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 w ater-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 a naerobiosis were most closely related to the wet and mesic meadow vegetatio n 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 vegetati on types. Indicator species associated with particular water-table regimes were identified for each vegetation type based on two-way ordered tables. C arer nebrascensis, an obligate riparian species, occurred at water-tables o f 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 th ese types could be used only to indicate broad ranges in water-table depth. Integrative environmental variables that incorporated the temporal variati on in water-tables (i.e., days that depth to water-table was less than 30 a nd 70 cm; degree days of anaerobiosis; range in water-table depth during th e growing season) demonstrated closer relationships to the vegetation types than water-table alone. They were also more sensitive to the spatial and t emporal differences in water-tables than individual plant species or vegeta tion types. Environmental and integrative environmental variables may respo nd more quickly to changes in local hydrology than plant species and are po ssibly more sensitive indicators of both current water-table status and pot ential vegetation.