Some Effects of Grazing Intensity on Bitterbrush Dispersion

Abstract

Three fence-line comparisons were made to determine the effect of grazing intensity on the density and dispersion of bitterbrush (Purshia tridentata (Pursh) DC.) in a uniform habitat. Bitterbrush density was reduced by heavy grazing but was not affected by moderate use. The large increases in mean area per plant that occurred under heavy grazing did not alter the overall form of random population dispersion. In a comparison of heavy versus moderate use, inclusion of 1-year-old plants on the moderately grazed area (10% of population) caused aggregation of the population. Even though a larger number of 1-year-old plants (22% of population) occurred in the heavily grazed comparison, the population remained random. The contrasting reaction probably resulted from a differential pattern of seedling mortality due to different amounts of herbaceous understory in the two shrub populations.