Plant communities growing on boulders in the Allegheny National Forest: Evidence for boulders as refugia from deer and as a bioassay of overbrowsing

  • Carson W
  • Banta J
  • Royo A
 et al. 
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Abstract

Deer have been overabundant throughout much of Pennsylvania since at least the 1940's. We compared plant communities in the Allegheny National Forest (ANF) on boulder tops and the forest floor to test the hypothesis that large boulders serve as refugia for plants threatened by deer herbivory. Five of the ten most common woody species (hemlock, Tsuga canadensis L., mountain maple, Acer spicatum Lam., red maple, A. rubrum L., striped maple, A. pensylvanicum L., and yellow birch, Betula alleghaniensis Britton) occurred at much higher densities on boulders than in randomly selected areas of the same size adjacent to these boulders on the soil surface. We never encountered any individuals of hemlock, mountain maple, or red maple on the forest floor. Total woody species density (excluding root suckers of beech, Fagus grandifolia Ehrh.) was nearly three times higher on the boulders. Woody species richness and evenness, as well as forb and shrub cover and richness, were also much greater on boulders. Our results strongly suggest that overbrowsing by deer can dramatically reduce tree regeneration and diversity as well as reduce forb and shrub abundance in the ANF. Furthermore, understory plant communities are now dominated by species that are known to be unpalatable or tolerant of deer browse, particularly beech, grasses, and ferns (Dryopteris carthusiana Vill. and Thelypteris noveboracensis L.). The primary alternative explanation is that conditions on the surface of boulders are superior for numerous woody and herbaceous species. Although we cannot rule this out, we consider this alternative improbable because of the poor nature of these habitats. Because deer are reducing the diversity and abundance of both woody and herbaceous species, we conclude that deer are damaging the very nature of these hardwood forests. Furthermore, our findings suggest that boulders could serve as an inexpensive bioassay of deer impact on vegetation whenever they are common and large enough. Boulders may also overcome the problem associated with the ghost of herbivory past, whereby plant species that would respond to the reduction of herbivores are absent because they have been driven to very low abundance or local extirpation. Our findings suggest that sampling the surface of boulders circumvents this problem because the vulnerable species were never subjected to sustained browsing on these refugia; consequently, these areas may represent typical levels of abundance of these vulnerable plant species.

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Authors

  • W P Carson

  • J a Banta

  • a a Royo

  • C Kirschbaum

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