In forested ecosystems, canopy tree species can modify biotic and abiotic environmental factors that, in turn, influence recruitment dynamics of woody vegetation. Such effects on regenerative dynamics of forest vegetation have been hypothesized to account for the coexistence of tree species, especially in mesic temperate forests co-dominated by American beech (Fagus grandifolia) and sugar maple (Acer saccharum). While several studies have examined the effects of canopy adults on juvenile and sapling stages of beech and sugar maple, less is known about canopy effects on seedling dynamics and how these might be modified by the fragmented nature of forests. We hypothesized that recruitment dynamics of beech and sugar maple differ between habitats beneath conspecific and heterospecific adult trees and vary with distance to the forest edge. In each of three fragmented forest stands in northeastern Ohio, we measured density and stem size of seedlings and beech root sprouts <30 cm tall during the 2007 growing season. Plants were sampled in sixty 20-m2 circular plots positioned beneath beech and sugar maple adult trees (>45 cm DBH), and in thirty randomly placed plots >10 m away from adult trees. We also measured canopy openness above plots and plot distance to the forest edge. Results/Conclusions
Mean density and stem size of both beech and sugar maple were equivalent among habitats. However, both species' mean density increased with canopy openness and distance to the forest edge. In addition, mean ratios of sugar maple stem height to stem basal diameter were positively associated with distance to the edge. By contrast, mean density of beech root sprouts was highest under beech adults and was not associated with environmental factors. We conclude that the localized establishment of beech and sugar maple seedlings is most influenced by environmental factors that are unrelated to the location and species identities of conspecific and heterospecific adult trees. Environmental factors such as light and wind can shape species recruitment by affecting the growth and survival of young plants or by altering the production, dispersal, and germination of seeds. Moreover, these factors—and their subsequent effects on seed and seedling dynamics—can be modified by habitat fragmentation and other disturbances and can result in the favored success of one species. In this way, differential species responses to environmental factors—not canopy effects—at young life history stages may mediate the co-dominance of American beech and sugar maple.