Elk browsing intensity impacts aspen recruitment on multiple use landscape over three decades

Abstract

Quaking aspen (Populus tremuloides) stands are vital to ecosystem health, providing wildlife habitat, water and carbon sequestration, and natural fuel breaks. Aspen stands occupy approximately 1.4% of the Greater Yellowstone Ecosystem, which includes the Northern Yellowstone Winter Range, an important ungulate wintering area. Historically, aspen stands occupied approximately 4-6% of the Northern Yellowstone Winter Range. In 1994, the Northern Yellowstone Winter Range elk population peaked at ~19,000 individuals and aspen coverage on the Northern Yellowstone Winter Range declined to 1% due to overbrowsing by elk. As of 2024, the elk population has decreased 71% since its peak, and 85% of the elk herd now migrates north of Yellowstone National Park (YNP) on the Custer Gallatin National Forest (CGNF) in the winter. Because new stems are vulnerable to ungulate browsing, stand sustainability depends on larger stems that are more resistant to herbivory. Assessing the health of Greater Yellowstone Ecosystem aspen therefore requires monitoring stand size structure. The goal of this study was to prioritize aspen management on the CGNF to sustain aspen stands long-term based on outcomes of repeated measurements of over 300 aspen plots on the CGNF north of YNP that were established in 1990. Previous studies concluded that aspen recruitment was increasing on a local scale in a few surveyed drainages, but was not increasing on a landscape scale. It was hypothesized that more time may be needed for aspen recruitment to increase in response to the decline in elk. Even though overall elk density has declined, elk density, and thus browsing pressure, has increased on the border of YNP due to the Northern Yellowstone Winter Range elk migration shift. I found that aspen stem recruitment is now increasing on a landscape scale, with a 90% increase since 2006, and that 59% of stands are considered sustainable long-term. Stems that have outgrown the upper browse height of elk (2 m) increased the most in aspen stands farther from YNP, at lower browsing pressure, and with greater snow water equivalent. Using this information, I prioritized aspen stands that would benefit from Forest Service management to ensure long-term aspen presence on the CGNF. This study is consistent with the hypothesis that aspen stand recruitment is responsive to shifts in browsing pressure at a local and landscape level, as well as other aspen recruitment studies within YNP that found that elk overbrowsing due to increased elk density, led to aspen decline on the Northern Yellowstone Winter Range. An elk density of 3 elk/km 2 may be the threshold on the border of YNP for aspen recruitment to increase. If elk density rebounds to historic levels, this may result in a reversal of aspen recruitment in the future.