Understanding the potential effect apex predators may have on riparian plant communities, via a trophic cascade, represents an important research challenge in Rocky Mountain ecosystems. In the northern ungulate winter range of Yellowstone National Park where grey wolves (Canis lupus) were historically present, absent for seven decades, and recently reintroduced, our objective was to evaluate patterns of cottonwood (Populus spp.) recruitment for two adjacent reaches of the Lamar Valley. Results indicated that recruitment was common in both reaches when wolves were historically present and declined because of intensive herbivory from elk (Cervus elaphus) after wolves were extirpated in the early 1900s. By the 1970s, cottonwood recruitment along both reaches had essentially ceased. Wolves were reintroduced in the mid-1990s, and by 2012, some 4660 young cottonwoods ≥2 m in height (the general upper browse level of elk) had become established within the 2-km-long upper Lamar study reach, consistent with re-establishment of a tri-trophic cascade involving wolves, elk, and cottonwoods. However, within the 8-km-long lower Lamar study reach, only 22 young cottonwoods had attained a height of ≥2 m because of high levels of herbivory, especially from bison (Bison bison). Top–down trophic interactions involving wolves and elk, as well as reach characteristics and browsing by bison, appear to explain the strongly contrasting patterns of recent riparian cottonwood recruitment currently underway in the northern Yellowstone – one reach represented by a recovering riparian ecosystem and the other an alternative stable state with highly altered riparian vegetation and channel conditions. Copyright © 2014 John Wiley & Sons, Ltd.
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