Riparian vegetation of the Montana Yellowstone and cattle grazing impacts thereon

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Montana State University - Bozeman, College of Letters & Science


The objects of my research were two. To describe ungrazed vegetation of thirty sites well dispersed along the 500 mile length of the Yellowstone River. And to measure the effects of gazing on this vegetation by describing/comparing vegetation of these ungrazed 'control' sites with the vegetation of nearby grazed sites. Vegetation of the Yellowstone consists of three lateral bands on open shore (gravel or sandbar), willow thicket, and cottonwood forest. Their appearance on successively older deposits suggests control both by decreasing water availability (greater depth to water on inland sites with 'over deposits') and increasing age (overtopping, first by willow and then by cottonwood, and accumulation of shrubs). The primary longitudinal (downstream) change between foothill and plains sites, probably driven by decreasing rainfall, was the change of forest dominant from P. angustifolia to P. deltoides. The apparent failure of P. deltoides reproduction could eventually eliminate the forest zone. Grazing affects all of the five communities identified. With grazing, overall cover decreased in every vegetation type, with the greatest losses in sandbars, willow thickets and P angustifolia forests. Cottonwood seedlings were grazed on bars and in willow thickets.
Cover of hydric (Salix spp., Cornus, Ribes spp.) and mesic (Symphoricarpos) native shrubs was significantly reduced. Rosa spp. lost cover with trampling. Invasive shrubs Russian olive and tamarisk, downstream, are apparently unaffected by grazing, except indirectly as grazing reduces their competition. Forb cover was most affected in Populus forests, with natives declining and non-natives increasing with grazing. Non-native grasses, the dominant herbs in cottonwoods, become more dominant with grazing. Observation of the exotics present emphasized the equal or greater impact of their invasion. Cover of exotics rises laterally from shore (50% with equal grass/forb composition) through willow thicket (70%, mostly exotic grass) to cottonwood forest (76-78%). The diversity of exotic herbs increases (laterally and downstream.). In the shrub layer Russian olive and tamarisk, pests in the American Southwest, increased greatly in the Plains in the 1980-2000 period and have a potential to dominate the forest and willow zones respectively. The ecosystem impacts of exotic increase will likely modify aesthetics, wildlife, and ranching drastically.




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