Tree seedling establishment under insect herbivory: edge effects and interannual variation

As the density and species composition of insects: may change in relation t o distance from the forest edge, the role of herbivory in tree establishmen t may also change across edges. To determine the importance of insect herbi vory in tree establishment, insect densities were experimentally altered at different distances, from the forest edge. Plots were established at three distances from the edge, with plots located in forest. edge, and field hab itats. In half of each plot, insect densities were reduced by insecticide a pplication. Seeds of two tree species, Acer rubrum and Fraxinus americana, were planted into each plot in 1995. The experiment was repeated in 1996 wi th the addition of Quercus palustris and Quercus rubra. Distance from the forest edge was the most important factor in determining seedling emergence and mortality. Overall seedling performance increased fr om held to edge to woods, although responses varied among species. In 1995, a drought year, insect removal increased emergence and decreased mortality of tree seedlings. In 1996, a year with normal precipitation, insect remov al had much less effect on A. rubrum and F. americana. For the two Quercus species, mortality was reduced by insect removal. The tree species: differe d in their susceptibility to insect herbivory, with Acer rubrum the most su sceptible and Fraxinus americana the least. Herbivory by insects was shown to have the potential to affect both the composition and spatial pattern of tree invasions. Herbivore importance differed greatly between the two year s of the study, making the interaction between insects and tree seedlings v ariable both in space and time.