Invasive earthworms effect on northern hardwood forest productivity

Background/Question/Methods

During the last Ice Age earthworms were eradicated from the Great Lakes Region. Earthworms present today are invasive, were introduced by European settlers, and have since spread via migration and widespread dispersal by people. Earthworms alter the structure, composition, and nutrient cycling of forest communities. We examined how earthworm invasion effects tree growth and productivity in a northern hardwood forest. We sampled earthworm populations along a 400m transect that crossed an active earthworm invasion front in the Chequamegon National Forest in northern Wisconsin, using mustard extraction techniques and collected tree-ring samples from 10 sugar maple trees at each earthworm sampling location. Tree-ring samples were compared with other chronologies and climate data. Root samples were taken 5 sugar maple trees at each earthworm sampling location to be analyzed for changes in root anatomy as earthworms moved through the study site.

Results/Conclusions

Earthworm biomass increased with distance along the transect. The species composition of earthworms also varied across the transect.  Ring widths were significantly wider in earthworm-free conditions. The forest was significantly more productive in the earthworm free sites compared to earthworm-invaded sites. Correlations among the chronologies for each sampling site over the last 40 years were higher during wet years (r=0.83, n=20) than the dry years (r=0.52) indicating that earthworms are altering the forests response to climate. Specifically earthworms create a more drought sensitive forest. Additionally there is evidence that calls into question the direction of succession within the forest.  In the earthworm free site there are a large number of maple saplings. Across the active invasion front saplings (2-5cm) had fallen over as the soil and organic layer had been eaten out from under them. In the heavily invaded location there are no maple saplings and pine regeneration was found under a maple canopy. This illustrates the influence of earthworms on forest productivity, climate-tree growth relationships, and forest composition.