Both native and invasive species that form forest understory layers affect tree seedling establishment worldwide. We compared the density and survival of canopy tree seedlings under and outside patches of the native shrub, Asimina triloba (L.) Dunal (Annonaceae) (pawpaw). We also conducted a manipulative experiment to determine whether above ground or below ground competition was more important in seedling growth and survival. Above ground competition was manipulated by tying back pawpaw stems and below ground competition by trenching the perimeter of the study plots. Overstory density (measured by a canopy densiometer) was greater under pawpaw than outside pawpaw. Tree seedling density was approximately three times greater outside pawpaw than under pawpaw over the range of sites. Seedlings under pawpaw were both younger and shorter than those outside of pawpaw. Survival varied by species. Acer saccharum seedlings were about one and a half times more likely to survive outside pawpaw than under pawpaw. Prunus serotina seedlings were about three times more likely to survive outside pawpaw than under pawpaw. In contrast, pawpaw did not affect the survival of Fraxinus spp. seedlings. The combination of above ground and below ground factors was more important in the survival and growth (measured by biomass) of planted A. saccharum seedlings under pawpaw than either above ground or below ground factors alone. Given that conditions such as elevated deer herbivory and tree diseases may provide opportunities for pawpaw to expand, continued attention to the pawpaw understory is warranted.
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Vol. 75 • No. 4