Deer Browsing Creates Rock Refugia Gardens on Large Boulders in the Allegheny National Forest, Pennsylvania

Abstract

We surveyed the vegetation growing on the surface of tall boulders (>1.5 m), on the surface of short boulders (<0.5 m) and in areas adjacent to these boulders on the soil surface in the Allegheny National Forest, Pennsylvania. Our purpose was to test the hypothesis that tall boulders serve as refugia from deer browsing for both woody plant species as well as understory herbs. We conducted our surveys during spring when many of these herbs reproduce sexually and their inflorescences are vulnerable to browsing. Deer cannot gain access and browse on these tall boulders, whereas they can easily browse plant species on shorter boulders and on the forest floor. We predicted that the tall boulders would have the highest plant diversity, woody species density and the density of reproductive individuals of understory herbs. We found a total of more than 1300 reproductive individuals of flowering herbs of more than 10 species growing on tall boulders and only 6 reproductive individuals of only a few species growing on the soil surface. Overall, there was a much greater density and diversity of woody species and sexually reproducing herbs on tall boulders or Rock Refugia Gardens than either on short boulders or the soil surface. Because the diversity and abundance of vegetation on the short boulders was nearly identical to that on the forest floor, our findings are unlikely to be explained by tall boulders providing a superior edaphic or abiotic habitat relative to habitats on the forest floor. Species accumulation curves showed that plant species continued to accrue with area on tall boulders, but did not increase with area on the forest floor or on the short boulders. These Rock Refugia Gardens demonstrate the pervasive and deleterious impact that deer are having on plant species diversity and forest regeneration in the Allegheny National Forest and likely elsewhere in the eastern US where these vertebrates are abundant.