Plant Species Composition and Groundwater Levels in a Platte River Wet Meadow

Species composition was monitored in seven permanent exclosures at Mormon Island Crane Meadows, near Grand Island, Nebraska from 1982 to 1987. Crane meadows is a 1,050 ha (2,600 acre) native lowland prairie complex with a corrugated topography of wetland swales and dry sand ridges. Variable precipitation, periodic over-bank flooding, and river stage fluctuations complicate the system\u27s hydrology. In general, springs are wet, but, by late summer, the meadows are usually dry, closely paralleling river flows and precipitation patterns. A sustained high water period in 1983 and 1984 was responsible for major changes in species abundance. Plant responses were consistent with species distributions along the topographic moisture gradient. Species at the lower end of the gradient were subjected to the greatest fluctuation in moisture and responded the most. In wetland swales sedge (Carex aquatilis Wahl.) , + 37%; spikerush [Eleocharis obtuse(Willd.) I.A. Schult.], + 36%; and American bulrush (Scirpus americanus Pers.), + 14%; increased significantly during the high water period. At moderate elevations indiangrass [Sorghastrum nutans (L.) Nash], + 22%; and switchgrass (Panicum virgatum L.), + 24%; increased, but big bluestem (Andropogon gerardii Vitman), -28%; showed a significant decline. All species except switchgrass returned to their former cover when water levels declined. Groundwater levels (mean and maximum) were the most important environmental parameters associated with changes in species cover. Switchgrass, however, did not respond directly to groundwater levels. High water conditions probably promoted the expansion of this species but it was able to sustain itself under much lower subsequent moisture conditions. Big bluestem and switchgrass also increased significantly with prescribed burning