Shrubsteppe ecosystems in the Intermountain West have suffered extreme alteration from a variety of factors. Using a retrospective approach, I studied the effects of horse and cattle grazing at the northern edge of the range in southern British Columbia, Canada, where the shrubsteppe is not as heavily altered and ungrazed sites remain in areas dominated by antelope bitterbrush (Purshia tridentata). I measured shrub and understory cover at 10 sites that were either ungrazed, lightly grazed, or heavily grazed. Cover of antelope bitterbrush decreased with grazing, and cover of big sagebrush (Artemisia tridentata) increased with grazing intensity. I sampled 72 species of vascular plants in the understory. Livestock grazing resulted in more bare soil, especially at sandy rather than rocky sites, and in quadrats located in the interspaces between shrubs. More bare soil was associated with less spikemoss (Selaginella spp.) and less microbiotic crust cover. Of the 3 most common bunchgrasses, sand dropseed (Sporobolus cryptandrus) was associated with more bare soil but only at sites without spikemoss. Red three-awn (Aristida purpurea var. longiseta), which grew best without litter or microbiotic crust, was most commonly found with spikemoss. Needle-and-thread grass (Hesperostipa comata), the most palatable abundant bunchgrass, was affected by livestock grazing, with shrub canopy cover offering some protection from grazers at the most heavily grazed sites. Rangeland management prescriptions in this area should take soil differences into account, with sandy soils being more prone to overgrazing and disturbance of the microbiotic crust cover than rocky soils.
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Vol. 68 • No. 2