Olson et al. complied data over a 25-year period, at the Livestock and Range Research Station in Montana, to describe quantitative changes in basal cover of vegetation in response to variation in precipitation amount and grazing intensity. Olson et al. used multiple regression analysis with basal cover as a dependent variable and precipitation parameters as independent variables to develop predictive equations. Basal cover values that were predicted for each species were used to create response surfaces which provided an effective means of interpreting species response to precipitation combinations. Three general responses were exhibited by most species although individual variation between response surfaces occurred. The most frequent response was an increase in cover under either very high or very low precipitation regimes. The second general type of response curve shows a direct relationship between cover and precipitation, regardless of the year in which the precipitation occurred. The third general type of response curve includes species which respond only to precipitation from either the previous or current year. Some species had response curves that were similar for all grazing intensities, while others showed a difference in their response to precipitation under different grazing intensities. This indicated that grazing pressure played a more important role in the response of such species. Moderate grazing intensity appears to be most conducive for maintaining vegetative cover that is desirable for livestock production. However, the authors caution that stocking rate changes need to be anticipated and planned to coincide with available forage because of large fluctuations in cover due to varying precipitation.
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