Composition and tree-size distributions of the Snyder-Middleswarth old-growth forest, Snyder County, Pennsylvania

Castanea, Mar 2003 by Zawadzkas, Peter P, Abrahamson, Warren G

White-tail deer browsing is currently among the most important agents of forest disturbance in the eastern United States. In Pennsylvania overbrowsing dates to the 1930s and its impacts are evident even in old-growth areas such as the hemlock-white pine-northern hardwoods at Heart's Content (Whitney 1984, Rooney and Dress 1997). Such browsing can shift the balance of canopy species because of differential recruitment by less-preferred and often light-requiring species such as red maple, black birch, and yellow birch (Whitney 1984, 1990; Rooney 1995; Abrams et al. 2000, 2001). There is evidence of deer use (droppings) and browse within the Snyder-Middleswarth old-growth forest. The gap in the smaller size classes of chestnut oak may be a consequence of juvenile mortality due to deer browsing. However, we did not see the same gap in smaller size classes with hemlock, a species that can be used as winter browse by deer in areas with high deer densities (Anderson and Loucks 1979). On the other hand, small hemlock stems only a few cm in diameter can be more than a century old (M.D. Abrams pers. comm.). Consequently, even though small individuals of hemlock are present, it is possible that there has been limited hemlock recruitment for several decades.

The continued domination of the Snyder-Middleswarth old-growth forest by hemlock could also be appreciably impacted by an outbreak of the hemlock woolly adelgid. This exotic herbivore was first reported in southeastern Pennsylvania in the late 1960s (McClure 1987) and it is present in central Pennsylvania (Abrahamson, pers. obs.). Should appreciable hemlock mortality occur, black birch would likely gain considerable importance (Orwig et al. 1998). Other herbivores including the elongate hemlock scale, spruce spider mite, hemlock rust mite, and cryptomeria scale could also threaten hemlock survival at the Snyder-Middleswarth Natural Area.

ACKNOWLEDGMENTS

We thank Jonathan Paliotta, Amy Whipple, Duane Griffin, Marc Abrams, Ben Marsh, Irene Kralick, and Norm Conrad for field assistance, technical support, or helpful advice. Permission to study this site was granted by the Pennsylvania Department of Conservation and Natural Resources, Bureau of Forestry. The Katherine Mabis McKenna Foundation and David Burpee Endowment of Bucknell University provided financial support. This study represents a portion of a senior thesis submitted for the B.S. degree in Environmental Studies at Bucknell University by P.P.Z.

LITERATURE CITED

ABRAMS, M.D. 1998. The red maple paradox. BioScience 48:355-364.

ABRAMS, M.D. and D.A. ORWIG. 1996. A 300-year history of disturbance and canopy recruitment for co-occurring white pine and hemlock on the Allegheny Plateau. J. Ecol. 84:353-363.

ABRAMS, M.D., C.A. COPENHEAVER, B.A. BLACK, and S. VAN DE GEVEL. 2001. Dendroecology and climatic impacts for a relict, old-growth, bog forest in the Ridge and Valley Province of central Pennsylvania. Can. J. Bot. 79:58-69.

ABRAMS, M.D., D.A. ORWIG, and T.E. DEMEO. 1995. Dendroecological analysis of successional dynamics for a presettlement-origin white-pine-mixed-oak forest in the southern Appalachians, USA. J. Ecol. 83:123-133.