Long-term impacts of deer exclosures on mixed-oak forest composition at the Valley Forge National Historical Park, Pennsylvania, USA

ABRAMS, M. D. AND S. E. JOHNSON (School of Forest Resources, Forest Resources Building, Penn State University, University Park, PA 16802). Long-term impacts of deer exclosures on mixed-oak forest composition at the Valley Forge National Historical Park, Pennsylvania, USA. J. Torrey Bot. Soc. 139: 167– 180. 2012.—Odocoileus virginianus Zimmerman (white-tailed deer) populations at Valley Forge National Historical Park in southeastern Pennsylvania have ranged from 70 to 93 deer per square km over the last decade. In 2010, we surveyed 30 fenced (to exclude deer) and unfenced paired plots established in 1992 to evaluate differences in woody and herbaceous vegetation. In addition, a broader collection and analysis of overstory and understory vegetation data were conducted in the two dry Quercus spp. forests and one mesic Liriodendron tulipifera L. forest that contain the fenced and unfenced plots. The understory of the Liriodendron stand had 72% cover of Microstegium vimineum, (Trin.) A. Camus, an exotic grass. Across all three stands, sapling and seedling densities were low to moderate (500 and 10,000 stems ha, respectively) and were primarily shade tolerant Acer rubrum L. and Nyssa sylvatica Marsh. The numbers of Quercus seedlings and saplings were low to nonexistent in all stands. Similarly, the unfenced plots had low densities of seedlings and saplings, especially Quercus. The 18 year-old fenced plots contained 33,133 seedlings ha, but lacked tree saplings. The fenced seedlings were dominated by A. rubrum, Sassafras albidum, (Nutt.) Nees, N. sylvatica and Fraxinus americana L. in two of the stands and Quercus montana L. in one of the Quercus stands. Shrub cover averaged 51% in the fenced plots (mainly Lindera, Viburnum, Ligustrum and Lonicera) and 5% in the unfenced plots. Moreover, Microstegium cover was very low in all fenced plots, possibly due to the high shrub cover. Fenced plots also had higher plant species diversity (Shannon Index and richness). The results of this study suggest that the long-term exclusion of O. virginianus resulted in increased tree seedling number, shrub cover and diversity, and low Microstegium cover, but was inconsistent in stimulating Quercus regeneration and sapling recruitment.