Effects of Cattle Grazing on Shore Vegetation of Fluctuating Water Level Reservoirs

Abstract

Shore vegetation around Lakes Oahe and Sakakawea, mainstem Missouri River reservoirs, is a mosaic of shifting plant populations that responds to a combination of limiting factors including water level fluctuations and cattle grazing. Shore vegetation is important as it provides some wildlife habitat and spawning habitat for certain fish species, reduces erosion, adds to the aesthetic value of the shore environment, and provides forage for cattle grazing. The present study was done in 1976 to document the effects of cattle grazing on shore vegetation at seven sites that were inundated for a prolonged period in 1975. Some of the dominant shore species were Horedeum jubatum, Agropyron smithii, A. repens, Chenopodium album, Iva xanthifolia, Kochia scoparia, Melilotus ssp., Polygonum achoreum, P. lapathifolium, Rumex crispus,, and Xanthium strumarium. Solanum rostratum and Grindelia squarrosa along with several other species were favored by grazing. Floristically, ungrazed plots were more similar than grazed plots when pairs of sites were compared. Total plant coverages inside exclosures at three sites on Pierre Shale-derived soils ranged from 54% to 97% and biomass values ranged from 458 g m-2 to 720 g m-2, while outside exclosures at the same sites total plant coverages ranged from 11% to 82% and biomass values were 19 g m-2 to 259 g m-2. At one site, on loess-derived substrate, total plant coverages inside and outside the exclosure were 116% and 77% respectively, and biomass values were 606 g m-2 inside and 210 g m-2 outside the exclosure. All three sites on Lake Sakakawea are on glacial till-derived substrates, and plant coverages ranged from 128% to 155% inside exclosures where biomass values were 478 g m-2 to 1,766 g m-2. Outside the exclosures the total plant coverages were 24% to 144% and biomass values were 15 g m-2 to 474 g m-2. Shore vegetation develops between periods of high water; thus annual fluctuations in water levels, along with cattle grazing as limiting factors, keep shore vegetation in an early seral stage. Minimizing both water level fluctuations and cattle grazing for a given reservoir during a given year, preferably a 2-year period, would permit considerably more shore vegetational development. This regimen, if rotated among the six reservoirs, would over a period of years, benefit development of shore vegetation.