Legacy forest structures in irregular shelterwoods differentially affect regeneration in a temperate hardwood forest

Foresters are faced with managing forests for a broad variety of societal demands such as wildlife habitat, aesthetics and site stability while still providing monetary value to landowners through timber harvest. Regeneration methods need to be developed that offer alternatives to traditional management for timber to help meet these goals. Regeneration responses in third-growth forest stands, or those following harvest of second-growth stands are not well-studied. Such forests are now widespread across temperate regions and may exhibit growth patterns different than those reported previously for regenerating second growth that originated on land abandoned from agriculture. There is a need for research that can lead to new methods or modification of existing practice. Our study examines legacy trees and the associated regeneration across a 25-year chronosequence of 34 irregular shelterwoods regenerated from second-growth forest in southern New England. We compared regeneration of the three most common regenerating tree species: red maple (Acer rubrum), black birch (Betula lenta), and red oak (Quercus rubra). We used regression analyses to examine changes through time in height growth of the tallest regeneration for each species as well as changes in total abundance. Similarly, regressions for annual height growth of regeneration in relation to legacy overstory basal area were compared for each species. Over time, self-thinning occurred in the regeneration of all focus species, but at different rates. Black birch thinned most dramatically through time, and the saplings that survived retained a high position in the canopy. Red oak self-thinned most slowly, and its tallest stems retained competitive height, although did not surpass black birch. As legacy overstory basal area increased, annual growth of red oak regeneration slowed. Stand dynamics patterns are therefore significantly different from those of second growth forests where oak is a more vigorous competitor in both number and growth as compared black birch and red maple. Our results suggest that resource managers need to both recognize these differences in stand development and consider the tradeoff between increasing legacy trees and decreases in growth rate of oak regeneration, as well as long-term effects of increased structure post-timber harvest.