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The article focuses on evaluating the evidence for a trophic cascade among wolves, elk, and willow conducted in the Yellowstone National Park. A suggested evidence for a trophic cascade is demonstrated by willow growth increasing when browsing is reduced, and after than before wolf reintroduction, and after the wolf reintroduction being greater than would be expected by alternative effects of climatic or hydrological factors. Applying stem sectioning and growth-ring measurement it was found out that willow growth in 2001 was related to browsing pressure in the winter preceding ring growth. The result of the study best support the theory that a trophic cascade among wolves, elk, and willow was brought by the indirect effects of wolves on the elk on the northern range.