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Effects of certain climatic factors on the productivity and availability of forages on the Ashnola bighorn winter ranges Harper, Frederick Eugene


The seasonal development and yields of four major plant communities, of the important winter ranges of California bighorn sheep (Ovis canadensis californiana Douglas) in the Ashnola region, and their relation to basic climatic factors, were studied from May 1967 through December 1968. The four communities were: 1) Da bunch grass community dominated by Agropyron spicatum, 2) a sod grass community dominated by Poa pratensis, 3) a tree understory community dominated by Calamagrostis rubescens and 4) a half-shrub community dominated by Artemisia frigida. The climatic variables measured were: ambient air temperature, precipitation, including snow, evaporation and wind. Run-off, soil moisture and soil temperature were also measured. To reduce climatic variations between communities, all study sites were located at approximately the same elevation and were close to one another. Virtually all forage production in the four communities, in the growing seasons of both 1967 and 1958, occurred before the end of July. After this time, due to shattering, leaching and decay, decreases in herbage weights, ranging from 23 to 35 percent occurred by autumn, and further losses, ranging from 7 to 36 percent occurred over winter. The mean growth rates were essentially the same in the two years, but the cessation of growth two weeks earlier in 1968, resulted in lower production than in 1967. The dates spring growth commenced coincided closely with the dates that mean daily temperatures rose to 42 degrees Fahrenheit, whereas the dates growth ceased coincided closely with the dates that available soil water was exhausted. Both ambient air temperatures and soil temperatures remained favourable for growth until early autumn. The length of the growing season is the main factor determining net productivity of the Ashnola bighorn winter ranges; temperatures determine the beginning of the season and soil moisture deficits terminate the season. Moreover because the growing season is relatively short, climatic influences are important factors determining how much of the forage produced will remain potentially available for overwintering bighorn.

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