Topping Stimulates Bitterbrush Twig Growth

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The response of old bitterbrush (Purshia tridentata) shrubs to topping was studied on winter range for deer in southwestern Idaho. Tops of 15 randomly selected shrubs at least 6 ft tall were cut off 3 ft above ground level; 15 similar shrubs were selected as a control. Current year's growth was sampled on each shrub before topping and annually thereafter for 4 years. All shrubs produced similar amounts of twig growth the year before treatment. The year after treatment, topped shrubs produced nearly 9 times as much twig growth as the control shrubs. In subsequent years, topped shrubs outproduced control shrubs but at a declining rate. Topping old shrubs may be a useful, though perhaps temporary, way to increase production of bitterbrush browse. On some mule deer (Odocoileus hemionus) winter ranges in the West, bitterbrush is virtually the sole browse species, but annual production of browse is often much below the potential for the site. Many bitterbrush stands have a few young plants but an abundance of old plants, which produce little new growth either as twigs or sprouts. On areas where there are few or no new bitterbrush plants, range managers have sought methods to stimulate old shrubs to increase twig production, especially near the ground where deer can reach them easily. Encouraging results of a limited preliminary trial of topping old bitterbrush shrubs about 3 ft above ground led us to undertake a larger study designed to obtain a quantitative measure of twig production following topping. That study is the subject of this paper. Bitterbrush shrubs in stagnant stands may be deliberately damaged by burning or by blading or dozing to induce new growth of sprouts. However, to rely on sprouting that results from top-killing is probably a risky way to increase production of forage. Bitterbrush sprouts readily after burning in eastern Idaho (Blaisdell and Mueggler 1956:366), central Oregon (Driscoll 1963: 820), and California (Nord 1965:324), but it sprouts rarely in the western Great Basin area (Billings 1952:257). Unfortunately, many sprouts from fire-killed bitterbrush eventually die. Sprouting may also be induced by complete or partial removal of tops, but mortality may be rather high (Blaisdell and Mueggler 1956:368). Young, vigorous plants produced new sprouts more commonly than older plants. Phillips (1964) reported that railing, dozing with a blade 1-2 ft above the ground, and rolling with a large log each increased production of bitterbrush twigs an estimated 300 percent the following season. However, production in the treated stands during the second and third seasons fell below that of adjacent, untreated stands. Treatments killed 13 percent of the shrubs and seriously injured many others. Personnel of Inyo National Forest in 1964 (unpublished) reported vigorous twig growth from 22 bitterbrush shrubs pruned to a 4-ft height the preceding November.