Large boulders provide refuge for browse-sensitive species and reveal a legacy of overbrowsing and shifting baselines

Prepared by Gillian G. Revenis, Ryan Utz, Rose-Marie Muzika, Thomas P. Diggins, and Walter P. Carson

Throughout the Eastern United States, white-tailed deer populations have risen far beyond historical sizes. Fluctuations in deer herd sizes are driven by the loss of their natural predators, like wolves and mountain lions, as well as human alterations of their natural habitats. While deer are native species that fundamentally shape their environments, too much of one species can impact other levels of the ecosystem. Since deer have specific eating preferences, many native plant species are limited by high densities of deer, allowing for non-preferred plants and invasive species to fill in the gaps. Managers often use deer fences to monitor the extent of deer damage, but their effectiveness is limited since recovery in degraded forests can take decades. In this study, we ask two major questions: 1. Can areas where deer cannot access (large boulders) serve as a safe haven (i.e. refugia) for vulnerable plant species? 2. How might plant recovery inside seven-year-old fences still be affected by the long-term deer overpopulation?

We investigated our research questions in an old-growth forest within the Huron Mountains in the Upper Peninsula of Michigan, US. While this forest has never been logged before, it has still been impacted by decades of high densities of white-tailed deer, up until recent years where populations have stabilized. In comparing plant communities on boulders to those on the forest floor (within and outside protected fences) we found that the tops of boulders had 60% more plant species and higher plant diversity. We also observed little differences between plant community composition inside and outside of fences on the forest floor, indicating that much more time is needed to see the reintroduction of vulnerable plants back into the understory.

Our findings suggest that refugia, like large boulders, serve as vital safe havens for plants that have largely disappeared from the forest floor due to decades of deer overpopulation. Boulders serve as isolated pockets of diversity, revealing what forests may have looked like before deer populations surged. While we may never be able to reconstruct the forests of the past, boulders offer a new baseline to illustrate the extent and legacy of deer damage.

This is a plain language summary for the paper of Revenis et al. (2025) published in Applied Vegetation Science (https://doi.org/10.1111/avsc.70021). The post was prepared by Gillian G. Revenis.