A Century of Overabundant Ungulates in Yellowstone's Lamar Valley: First Elk and Now Bison.

After decades of intensive herbivory by Rocky Mountain elk (Cervus canadensis), many deciduous plant communities across Yellowstone National Park's Northern Range began to recover following the mid-1990s reintroduction of gray wolves (Canis lupus), a trophic cascade. However, since the early 2000s, foraging pressure by the Park's North American bison (Bison bison) population has greatly increased. During this period, we evaluated the effects of an increased bison population on riparian plant communities in the Lamar Valley, an area of high bison use in the eastern portion of the Park's Northern Range. Using measurements of seedlings, saplings and small trees, and overstory trees, as well as chronosequence photographs, we studied the valley's cottonwoods (Populus spp.) and quaking aspen (P. tremuloides), both long-lived keystone species. Results indicated that cottonwood seedling heights were being suppressed by bison herbivory, preventing the establishment and growth of new cottonwood forests. The horning and rubbing effects of bison-accessible saplings and small trees of cottonwood and aspen were found to cause high levels of bark damage and mortality. The proportion of mature Lamar Valley cottonwood trees with bark damage from bison horning and rubbing increased from 5-6% to nearly 50% between 2001 and 2023, a period during which nearly one-third of these overstory trees had died. Our findings highlight how a century of unnaturally high levels of ungulate use, initially elk and now bison, has dramatically transformed the valley's riverine ecosystem. A recent management decision by the Park Service indicates the agency will maintain large numbers of bison into the future, thus continuing the disruption of riparian plant communities and ecosystem functions within the Lamar Valley, as well as other locations of high bison use in the Northern Range.