LINKING SURFACE AND GROUND WATER LEVELS TO RIPARIAN GRASSLAND SPECIES ALONG THE PLATTE RIVER IN CENTRAL NEBRASKA

Although the values of riparian grasslands are well established, few quantitative data are available linking the underlying hydrology to the riparian plant species that support these important communities. This lack of information is especially critical along the Platte River in central Nebraska where riparian grasslands support a multitude of migratory birds, and where flow re-regulation to enhance habitat for endangered species is being considered. Ecologists use a host of techniques to quantify how environmental gradients, like surface and ground water levels, are linked to plant species. Nearly all these techniques, however, produce results that represent gradients in general terms such as low to high elevation, xeric to mesic, and low to high concentration. While ecologists understand the implications of these imprecise scales, managers responsible for making decisions affecting one or more of these gradients need information that is more precise. For our study, we preserved the scale and units of a water-level gradient ranging from below the surface to above the surface by using non-linear equations to fit plant species response curves to this dominant gradient. Non-linear equations are more useful than linear equations, like polynomials, because their coefficients can be interpreted with a biological meaning such as population peak, optimum gradient position, and ecological amplitude. Plant response to this water-level gradient was fit to 65 species. From among eight water-level statistics evaluated, the peak 10-day running-mean water level for the growing season produced the best plant-response fit, suggesting that several consecutive days of high water levels are more influential than low water levels or the frequency of high water levels. Land management practices affected 21 species by changing their frequency of occurrence and/or their position along the water-level gradient. Most species have an ecological amplitude from 35-80 cm along the water-level gradient, but a few have amplitudes as narrow as 20 cm. For these species with narrow amplitudes, a small permanent change in the 10-day high water level might completely displace their topographic location, possibly causing their aerial extent to expand or contract depending upon the available topography.