A century of dramatically altered disturbance regimes has likely changed patterns of forest regeneration, with potential detrimental consequences for the future diversity, composition, and function of late-succession and old-growth forest stands. We quantified the diversity and composition of tree species in the canopy and understory layers of 19 late-successional and old-growth forest 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 of all stands was comprised of a small homogeneous subset of the canopy species. In addition, understory layers had lower species diversity than their respective canopies and showed a significant lack of species intermediate in shade tolerance. Oak species, which often require fire to regenerate, were common in the canopy of 12 stands, but nearly absent from the understory layer of all stands. Our findings are consistent for late-succession and old-growth stands located across a large geographic area of Pennsylvania and across stands of varying sizes, canopy species composition, and location in different physiographic provinces. These findings suggest that disruptions to historical disturbance and browsing regimes play a strong role in this dramatic homogenization of understory species composition. In the absence of large-scale disturbance, these stands will become relatively depauperate and composed of a homogeneous subset of species that are simultaneously shade-tolerant, browse-tolerant, and fire-intolerant.
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