Riparian forests of northwestern Pennsylvania's Allegheny High Plateau are often dominated by eastern hemlock, Tsuga canadensis, a tree species under threat from the introduced hemlock woolly adelgid (Adelges tsugae). It is vital to document the structure and composition of these forests before adelgid invasion to understand how they may respond to this disturbance and to provide information useful for future restoration and management efforts. To meet these goals, we compared the composition and structure of old-growth and second-growth stands in a T. canadensis-dominated riparian forest along Henry Run, Cook Forest State Park, Pennsylvania. Tsuga canadensis strongly dominated the large tree stratum of the old-growth stand, followed by Betula lenta and Fagus grandifolia. Tsuga canadensis and Pinus strobus were codominant in the second-growth stand with B. lenta, Fraxinus americana, and Acer rubrum as secondary species. Seedlings and saplings of dominant tree species in both old-growth and second-growth stands were under represented relative to small and large trees, indicating regeneration difficulties. The alien shrub Berberis thunbergii was the only sapling species to occur in the second-growth stand, perhaps a land-use legacy, but was absent from the seedling stratum. We predict that the potential loss of T. canadensis from the canopy at Henry Run may cause a shift in composition to B. lenta and/or P. strobus as these species capture gaps created by dead and dying T. canadensis. However, browsing pressure by white-tailed deer (Odocoileus virginianus) and competition with B. thunbergii may affect regeneration success of specific tree species and ultimately, future forest canopy composition.

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