Assessing the impacts of European earthworm invasions in beech-maple hardwood and aspen-fir boreal forests of the western Great Lakes region

In the Great Lakes region, little data exist on the distribution of exotic earthworm species or their relative impacts to soil structure and plant communities in forest types other than those dominated by sugar maple. Our objective was to document the earthworm populations in relation to their proximity to human development (e.g., campgrounds, boat landings, roads) in beech-maple dominated forests in Pictured Rocks National Lakeshore, Michigan, and aspen-fir forests in Voyageurs National Park, Minnesota; and to assess earthworm population relationships with soil characteristics and plant communities. Five earthworm species were commonly found in both forest types in three species assemblages also seen in other forests across the Great Lakes region. Total earthworm biomass supported by beech-maple forests was nearly half that reported in sugar maple dominated forests and the total earthworm biomass supported by aspen-fir forests was half that of the beech-maple forests. Distances to human development were poor predictors of earthworm biomass and species assemblage in general, but were correlated with the presence of Lumbricus terrestris. Earthworm richness and biomass were negatively associated with thickness of the Olitter and Oe horizons and positively associated with A horizon thickness. Increasing plant richness and changes in plant composition associated with earthworms in the beech-maple forests are likely related to stand conditions that simultaneously affect plant and earthworm populations rather than an earthworm effect. Comparisons to earthworm-free stands were not possible and are needed to further explore any potential causal relationships between earthworm and understory plant populations.