Human activities are increasingly introducing species outside of their traditional ranges and thus homogenizing the earth’s biological communities. Belowground ecosystems have not been spared the introduction of nonindigenous species and much of the eastern United States is undergoing an earthworm invasion that is having direct effects on plant species diversity. We hypothesized that root grazing by a nonindigenous earthworm species, Lumbricus terrestris (European nightcrawler) would have a more negative effect on the survival and biomass accumulation of an ectomycorrhizal tree species than an endomycorrhizal species. The effects of L. terrestris on Southern Appalachian tree seedling diversity were tested in a mesocosm experiment. Each mesocosm (30 x 37cm) was planted with 2 seedlings of Acer rubrum (red maple), Quercus rubra (red oak), or a combination of the two species. Each tree seedling treatment was replicated 40 times and 15 g of L. terrestris were added to 20 mesocosms of each seedling treatment resulting in 6 earthworm/tree seedling combinations. Soil sterilization and hand sorting were used to ensure that the control mesocosms were worm free at the start of the experiment. Tree seedling growth and survival was monitored for two years. In addition, several environmental variables were measured including soil pH and moisture, and photosynthetically active radiation (PAR, μmol/m2).
Results/Conclusions
At the conclusion of the experiment total aboveground biomass, total belowground biomass, leaf areas, litter accumulation, and earthworm numbers and dry weights were quantified. Final excavations recovered earthworms from some control mesocosms despite rigorous efforts to keep the controls earthworm-free. There were no significant differences in mean soil moisture (%), soil pH, and PAR among the treatments. After two growing seasons, the oaks grown in the presence of L. terrestris showed significantly less growth than those grown without earthworms. However, L. terrestris appeared to have little effect on the growth and survival of maples. Preliminary analyses show similar patterns for the above- and belowground biomass of the tree species. Overall, those mesocosms with L. terrestris had noticeably less surface leaf litter than the earthworm-free treatments. Our results suggest that introduced earthworms such as L. terrestris may influence long-term patterns of forest composition due to their differential effects on tree seedling survival.
