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1 March 2011 Aspen Recovery Since Wolf Reintroduction on the Northern Yellowstone Winter Range
David S Kimble, Daniel B Tyers, Jim Robison-Cox, Bok F Sowell
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Quaking aspen (Populus tremuloides Michx.) recruitment and overstory stem densities were sampled in 315 clones in 1991 and 2006 on 560 km2 of the Northern Yellowstone Winter Range (NYWR). A primary objective was to observe if aspen status had improved from 1991 to 2006: evidence of a wolf (Canis lupus) caused trophic cascade. Recruitment stems (height > 2 m and diameter at breast height < 5 cm) represent recent growth of aspen sprouts above elk (Cervus elaphus) browsing height, whereas overstory stems (all stems > 2 m) represent the cohort of stems, which will insure the sustainability of the clone. Overstory stem densities declined by 12% (P = 0.04) on the landscape scale when compared with paired t-tests. Overstory stems declined in 58% of individual clones and in 63% of the 24 drainages of the study area. The second objective was to determine which factors influenced changes in aspen density. Winter ungulate browsing (P = 0.0001), conifer establishment (P = 0.0001), and cattle (Bos spp.) grazing (P = 0.016) contributed to the decline in overstory stem densities when analyzed using a mixed effects model of log transformed medians. Eighty percent of the clones were classified as having medium to high browsing levels in 1991, whereas 65% of the clones received a similar rating in 2006, possibly due to the reduced NYWR elk population. Aspen recruitment has increased in some 2–10 km2 areas, but not consistently. Our study found that a trophic cascade of wolves, elk, and aspen, resulting in a landscape-level recovery of aspen, is not occurring at this time.

Society for Range Management
David S Kimble, Daniel B Tyers, Jim Robison-Cox, and Bok F Sowell "Aspen Recovery Since Wolf Reintroduction on the Northern Yellowstone Winter Range," Rangeland Ecology and Management 64(2), 119-130, (1 March 2011).
Received: 27 January 2010; Accepted: 1 December 2010; Published: 1 March 2011

Cervus elaphus
Populus tremuloides
trophic cascade
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