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Frelich, L.E. and A.R. Holdsworth. 2002. Exotic earthworms in Minnesota hardwood forests: an investigation of earthworm distribution, understory plan communities, and forest floor dynamics in northern hardwood forests. Final report submitted to the Department of Natural Resources. 15 pp.

Introduction:

For over 12,000 years Minnesota's forests developed without the influence of earthworms. If North America earthworm species ever inhabited Minnesota, they were extirpated during the last glaciation. Without earthworms, forests' fallen leaves accumulated and developed a thick duff layer that provided an excellent rooting zone for herbs and tree seedlings. Currently, over fifteen species of European earthworms inhabit Minnesota. Over the last 150 years European earthworms were likely accidentally and intentionally introduced with the importation of plant material and soils from Europe and the use of worms as fishing bait across the region. Ongoing studies suggest that invasive European earthworms have a notable effect on forest understory plan diversity and composition, nutrient cycling, and soil properties. Since the 1980's forest managers on the Chippewa National Forest have been concerned about the loss of understory plan cover and diversity in areas with high earthworm populations. Exotic earthworms are considered a major factor in the population decline of the state threatened goblin fern (Botrychium mormo).

At least seven different European earthworm species inhabit Minnesota's maple-basswood forests. Earthworm species are often divided into three broad ecological groups: epigeic, endogeic, and anecic species.

We know very little about the distribution of different earthworm species in Minnesota forests and the extent of impacts they may be having on forest plant communities. We also do not know why some forests seem to lose their forest floors, herbs and tree seedlings in response to earthworm invasion, while others do not. There are numerous factors that could influence how much understory plant communities are affected by the invasion of European earthworms. These factors included litter quality and quantity in the stand (directly related to forest composition and productivity), species and biomass of earthworms present, levels of deer herbivory, soil type, time since earthworms invaded, climate, and disturbance history. In a given stand, litter inputs are a major limiting factor for litter consuming earthworm populations. Litter in a more productive forest with greater litter fall rates is more likely to produce "surplus" litter that surpasses the earthworm community's ability to consume all litter each year. If this litter is of lower quality (higher carbon:nitrogen, lignin:nitrogen ratios, etc.), and less preferred by earthworms, then the forest floor is more likely to persist year round. If the major litter consuming earthworm, L. terrestris, is absent from the stand, the forest floor is even more likely to persist year round.

    This report describes two studies of earthworms in Minnesota hardwood forests:
  1. A regional survey of the distribution of earthworms in lakeside mature maple-basswood forests and how this distribution relates to understory plan diversity and cover.
  2. An experimental study on the effect of leaf litter type and quantity on the rate of forest floor decay in mature maple-basswood stands with different earthworm communities.

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