EFFECTS OF RODENTS ON SURVIVAL OF TREE SEEDS AND SEEDLINGS INVADING OLD FIELDS

Abstract

Tree invasion of old fields is an important process affecting landscape het- erogeneity and species diversity, as well as the quality of wildlife habitat, in eastern North America. We examined the effects of tree seed and seedling predation by small mammals on the rate, spatial pattern, and species composition of tree invasion in old fields. We performed field experiments inside U-shaped enclosures built at forest-field edges, within which we manipulated the density of meadow voles (Microtus pennsylvanicus), which are major seedling predators. Although we did not directly manipulate density of white-footed mice (Peromyscus leucopus), which are major seed predators, mouse density was high wherever vole density was low. We introduced cohorts of seeds and seedlings of red maple (Acer rubrum) or sugar maple (Acer saccharum) (as specified in the Methods for each trial), white ash (Fraxinus americana), white pine (Pinus strobus), and tree-of-heaven (Ailanthus altissima), all common early invaders of old fields in our study area. Rates of seedling predation were higher in enclosures with higher density of voles, and voles showed distinct preferences for red maple, white ash, and Ailanthus over white pine. Predation rates on small seedlings were higher than on larger seedlings. Seedlings near the forest edge were killed at a lower rate than those >5 m into the field. Predation rates on seeds were higher in low-vole-density treatments, where mice were more abundant. Mice showed distinct preferences for white pine, avoided Ailanthus, and consumed sugar maple and white ash at intermediate rates. Predation rates on seeds were higher in shrub-dominated than in herbaceous old fields. Overall density of mice increased dramatically between 1994 and 1995, and seed predation rates increased concomitantly. In 1994, seed predation was most intense 5 m and 10 m into the field, and less intense at the forest-field edge and 20 m into the field; although a similar pattern was observed in 1995, the variation was not statistically significant. We conclude that both of these common small mammal species can have pro- found effects on survival of tree propagules near forest-field edges, but that their effects are strongly dependent on density. Voles had a strong, negative direct effect on seedling survival, but a positive indirect effect on seed survival by inhibiting seed predation by mice. Voles and mice: (1) showed distinctly different food preferences; (2) had their strong- est effects at different positions with respect to the forest-field edge; and (3) had their strongest effects in different old-field types (herbaceous fields for voles, shrub-dominated fields for mice). These two species are not functionally redundant in their effects on old- field succession.