An increase in herbivory of cottonwood in Yellowstone National Park
Keigley, Richard B.
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This study examined an effect of elk (Cervus elaphus nelsoni) on narrowleaf cottonwood (Populus angustifolia) in northern Yellowstone National Park, where stands consist of old trees and younger, densely-branched bushes. The elk herd increased from a census of 3,172 in 1968 to a census of 18,913 in 1988. The purposes of this study were to: 1) document the height-growth of cottonwood bushes, 2) determine if the height of browsing corresponded with snow depth, and 3) determine if there has been a recent increase in cottonwood herbivory. In 5 stands of different age (ranging ca. 9-45 y old), I measured the height of live previous-year-growth and the height of the oldest stems killed by browsing. The tallest previous-year-growth was 80 cm; all stems taller than 29 cm had been browsed. Stems were killed by browsing closer to the ground in younger stands (respectively, 87, 62, 28, 14, and 9 cm; P lt 0.001). There was no change in mid-winter snow depth during the period 1950-1994. The 2 stands established since 1977 had relatively small variances in the height at which stems were killed by browsing (21 and 15 cm-2), a uniformity likely caused by intense herbivory since respective stand creation. The large variances in the height of browse-killed stems in older stands (745, 399, and 291 cm-2) were likely caused by an initial period of light-to-moderate herbivory followed by an increase in herbivory that killed the stem tips at the heights existing at the time. The bush growth-form apparently results from an increase in herbivory that occurred between 1968 and 1977, a period in which the elk winter census increased from 3,172 to 8,981. The weight of evidence suggests that EuroAmerican influences have caused the northern elk herd to increase in number since the establishment of the park. If herbivory does not decrease, cottonwood may be eliminated from Yellowstone's northern range