Large Boulders Provide Refuge for Browse-Sensitive Species and Reveal a Legacy of Overbrowsing and Shifting Baselines

Abstract

Question

Overbrowsing by abundant deer reshapes the composition of forests from palatable native species to browse-tolerant species, leading to simplified understories with reduced biodiversity. Here, we ask whether geologic refugia (e.g., tall boulders) will provide a haven for vulnerable plant species to escape browsing pressure from high populations of native deer.

Location

Huron Mountains, Michigan, USA.

Methods

We compared the percent cover, species richness, and Shannon diversity of all plant species on tall boulders to adjacent fenced exclosures and to forest floor reference plots to quantify the degree to which deer impact one of the largest remaining tracts of old-growth forests in the Eastern United States.

Results

We found that boulders contained 60% higher richness and diversity than reference plots, as well as a more diverse mix of midstory species, shrubs, and forbs. Seven-year-old exclosure and reference plots were nearly indistinguishable from each other and were dominated by Acer saccharum and other browse-tolerant species.

Conclusions

This study demonstrates that boulders are a highly effective, though rarely used, proxy or bioassay of the degree to which chronic overbrowsing creates impoverished understories composed of browse-tolerant or unpalatable species. Our findings highlight the significance of refugia as a haven from overbrowsing for vulnerable plant species and emphasize the importance of using exclosures alongside refugia.