(Montana State University - Bozeman, College of Letters & Science, 2005) Eggers, Margaret Joy Slack; Chairperson, Graduate Committee: Theodore Weaver
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.
