Patterns of shifting tree species composition and diversity loss in 19 old-growth forest stands in Pennsylvania

A century of fire suppression and overbrowsing by deer have likely altered patterns of forest regeneration, with detrimental consequences for the future diversity, composition, and function of old-growth stands. We quantified the diversity and composition of tree species in the canopy and understory layers of 19 old-growth stands in Pennsylvania to evaluate the consequences of existing regeneration patterns for the future composition of these communities.Despite relatively high canopy diversity across all stands, the understory composition of all stands converged to a homogeneous subset of the canopy species. In addition, understory layers had unusually low stem densities, lower species diversity than their respective canopies, and showed a significant lack of mid-tolerant species. Oak species, which often require fire to regenerate, were common in the canopy of 12 stands, but absent from the understory layer of all stands. Our findings suggest that deer browsing and fire suppression are partly responsible for this dramatic alteration of understory species composition. The lower diversity and homogenized species composition of the understory indicates that the future composition of these stands will not resemble the current composition. Alteration of tree species composition is likely to have detrimental effects on the survival of species dependent upon old-growth habitat