Assessing Long-Term Changes in Aspen Forest Cover in Northern Yellowstone National Park Before and After Carnivore Recovery

The recovery of large carnivores (wolves, bears, cougars) in Yellowstone National Park and the subsequent decline of the area's elk herd have raised key questions about ecosystem restoration—particularly for species such as aspen, which are heavily browsed by elk. Following wolf reintroduction in the mid-1990s, scientists began tracking changes in browsing intensity and height of young aspen within sampling plots, assuming reduced browsing and taller stems were signs of population recovery. However, no studies have measured whether the total area covered by aspen stands actually increased, as would be expected with broader recovery. Moreover, the potential influence of climate change was not always thoroughly investigated and was sometimes overlooked or minimized. My research addressed these issues by using aerial photographs to measure changes in aspen cover within 79 stands across northern Yellowstone from 1954-2022. I then examined whether cover changes from 1991–2022 were predicted by sampling plot measurements such as percent browse and stem height, and assessed the relative influence of percent browse compared to two climate variables—water deficit and monthly snowpack—on these changes. Overall, aspen cover declined significantly from 1954-2022. Surprisingly, the rate of cover loss accelerated after predator recovery (1991-2022), despite the elk population decreasing. Aspen recovery was patchy, with only 10% of aspen stands expanding. While plot measurements were slightly related to cover change, the variability in these relationships was high, indicating the influence of other factors. Percent browse had a stronger effect on cover than water deficit or snowpack, but that may change if the climate continues to warm. These findings demonstrate that the recovery of large carnivores does not guarantee widespread improvements for plants like aspen, particularly in a changing climate. The ecological effects are complex and vary significantly across the landscape; for example, stems may grow taller within localized refuges even as the overall stand cover contracts due to climate stress or browsing at its periphery. My study emphasizes the need to combine different types of information—broader-scale vegetation changes, small-scale plot data, and climate variables—to better understand how complex ecosystems respond to large predator recovery.