Revisiting trophic cascades and aspen recovery in northern Yellowstone

Abstract

We revisit the nature and extent of trophic cascades and quaking aspen (Populus tremuloides) recovery in the northern range of Yellowstone National Park, where studies have reported on Rocky Mountain elk (Cervus canadensis) browsing and young aspen heights following the 1995–96 reintroduction of gray wolves (Canis lupus). A recent study by Brice et al. (2021) expressed concerns about methodologies employed in earlier aspen studies and that results from those studies exaggerated the extent to which a trophic cascade has benefitted aspen, concerns such as: (a) the selection of aspen stands, (b) young aspen sampling and measurements within stands, (c) the upper browse level of elk, (d) cause of increased young aspen height growth, (e) interpretation of browsing and height data, and others. We individually address these concerns and conclude that earlier aspen studies have provided important insights regarding the recovery of aspen that is underway in northern Yellowstone and that they have not exaggerated the ongoing recovery. We found that Brice et al. (2021) misinterpreted or misrepresented various aspects of those earlier studies, while failing to address potential biases and shortcomings of their own 2007–2017 study, including; sampling aspen stands from only a portion of the park's northern range, not randomly selecting aspen stands across their study area, but only within identified treatments, varying sampling density (stands/km2) by more than an order of magnitude between treatments, and not sampling all stands in most years. Regardless of the research methodologies employed in various aspen studies, they have consistently shown that decreased browsing has resulted in greater young plant heights in the park's northern range, results consistent with an ongoing trophic cascade.