Invasive earthworm community composition linked to overstory vegetation across a climate gradient in western Minnesota

Background/Question/Methods  Minnesota’s forests were devoid of earthworms prior to European settlement in the mid-1800s.  Since then, European and Asian earthworms have spread into many parts of the state, but the extent and nature of this invasion is still only roughly documented.  In 2007, we described the worm populations in forested stands at six sites in west-central Minnesota.  Here we expand that work and report the characteristics and extent of earthworm communities across a transect running 400 km along the prairie-forest ecotone in western Minnesota and spanning a 2.5 °C (summer)-4.5 °C (winter) and 30 day growing season-length gradient.  At each of six forested sites we sampled worms and vegetation in four clusters chosen to represent four different forest environments: 1) & 2) high-light & low-light, oak dominated, and 3) & 4) high-light and low-light, sugar maple-basswood dominance.  Worm communities were sampled using the hot-mustard extraction technique and worms were measured and identified to species.  Vegetation was documented using 24m diameter mapped permanent plots. In this work, we asked whether the extent and composition of the invasive earthworm communities vary with 1) climate and 2) surrounding vegetation in forest stands growing at the edge of the eastern deciduous forest.

Results/Conclusions Earthworm biomass increases from north to south in our study transect, with total biomass per plot at our two southernmost sites more than double that found at our two northernmost sites.  At the stand level, invasive earthworm extent and community composition varies strongly with local tree composition.  Most prominently, worm biomass is negatively associated with bur oak (Quercus macrocarpa) basal area (kendall’s tau = -0.25, p = 0.002) and positively associated with basswood (Tilia americana) basal area (kendall’s tau = 0.33, p <0.001).  Sugar maple (Acer saccharum) dominated stands have abundant Aporrectodea sp. and Dendrobaena octaedra, consistent with reports from earthworm invaded forests further east in Minnesota.  The most abundant earthworm species under oak, on the other hand, is Octolasion tyrtaeum, a species previously listed as “uncommon” in the region.  Contrary to expectations from the literature, neither worm biomass nor the total abundance of any individual worm species is positively associated with the locally abundant invasive European buckthorn (Rhamnus cathartica) tree.  This lack of association is consistent, however, with results from previous work we have conducted in western Minnesota forests.