The Charter of the National Park Service requires conservation of park land, vegetation, and animals in essentially "natural conditions" for the enjoyment of people. However, the presence of people causes many changes in park ecosystems. Each park manager must evaluate the origin and condition of the present system. This paper examines the existing vegetation of Yosemite Valley to determine changes that have occurred in the recent past and the extent to which they might continue.
Since 1851, modern man has subjected the vegetation and soil in Yosemite Valley to burning, plowing, seeding of crops and pastures, irrigation, mowing, livestock grazing, clearing of trees and brush, logging, hand pulling of young conifer trees, and competition from introduced plants such as wild flowers, fruit and ornamental trees, exotic weeds, and many annuals. Changed drainage patterns and soil moisture conditions resulted from filling of low areas, fencing, road building, use of drainage tile in one of the meadows, lowering the Merced River bed at the valley outlet, burying utilities, and construction of hotels, stores, gravel pits, a sewage plant, and a slaughter house. Animal influences fluctuated with varying numbers of horses, dairy cattle, and beef cattle; tule elk from 1921 to 1933; and occasional reduction of deer, bear, rodents, and forest insects. The klamath-weed beetle (Chrysolina quadrigemina) was introduced into the valley in 1950.
This study of Yosemite Valley began in 1961 to (1) chronicle events that influenced the valley landscape from 1851 to the present (Gibbens and Heady 1964); (2) make a detailed soil survey of the valley (Zinke and Alexander 1963); (3) quantify the meadow vegetation (Ziegler and Heady 1964); (4) measure the forest vegetation (Ziegler and Heady 1965); and (5) collect information on recent vegetational changes. Results of the vegetational studies are presented here with reference to the soil and historical surveys without which vegetational patterns and successional changes would have little meaning.
Information on the impact of visitors aids landscape management. Yosemite particularly is suited to a study of man's influences because of several unique factors. Since Europeans first entered the valley in 1851, enhancement of aesthetic values and recreational use, rather than logging or grazing, has been the central objective (Bunnell 1911). Unpublished records maintained at the park headquarters permitted appraisal of events that caused vegetational change from the beginning of use. Photographs from the top of the canyon walls in 1866 are among the earliest oblique aerial photographs that recorded valley vegetation long before quantitative vegetational measurements were made. The canyon walls circumscribe environment, vegetation, and use of a small area with natural and unusually re strictive boundaries.
In this study we used current data on soil and vegetation; repeat photographs, some taken over 100 years apart; general descriptions, usually without quantifications of any kind; historical documents which often plead issues; and a large measure of deductive assessment to justify our conclusions on man's impact on the vegetation of Yosemite Valley.
Last Updated: 20-Nov-2007