Trophic cascades at multiple spatial scales shape recovery of young aspen in Yellowstone

Throughout much of the 20th century, the heights of young quaking aspen (Populus tremuloides) in Yellowstone National Park's northern ungulate winter range were suppressed due to intensive herbivory by Rocky Mountain elk (Cervus elaphus). However, following the 1995-96 reintroduction of gray wolves (Canis lupus), completing the park's large predator guild, young aspen in various portions of the northern range began to increase in height. From 1999 to 2015, a 17-yr period of declining elk densities in the northern range, browsing rates declined and young aspen heights increased once elk densities dropped below ∼4 elk/km2. The inverse relationship between browsing rate and young aspen height, a relationship linking elk and plants, was consistent with a re-established trophic cascade. Within the Glen Creek study area (8.3 km2), decreased browsing and increased heights of young aspen were associated, at least in part, with two hypothesized small-scale predation risk factors (i.e., escape impediment, view impediment). However, the young aspen height increases did not occur in the Mammoth study area (6.0 km2) and heights there remained short. With high levels of human activity at the Mammoth townsite, wolf activity near the townsite remained low, an example of “human shielding,” thereby allowing elk browsing to continue the suppression of young aspen. Overall, results indicated that Yellowstone's contemporary large predator guild, by altering elk behavior and density at several spatial scales, has not only contributed to a relatively widespread pattern of increased young aspen heights across much of the park's northern range, but also greater spatial variation in those heights.