Patterns of earthworm presence and potential influences on ecosystem structure and process in a second-growth northern hardwood forest

We have initiated a replicated, long-term, large-scale field experiment designed to test the effects of old-growth structural features on the biodiversity, productivity and nutrient cycling in a second-growth northern hardwood forest in northern Wisconsin. Based on observations from other north temperate forests, we recognize that the presence of exotic earthworms may also significantly influence ecosystem structure and function. The aim of this study is to assess the degree of invasion of our long-term study site prior to initiating experimental forest management. In our relatively undisturbed forested site, we expect that where worm densities are highest, the depth of the forest floor and the C:N ratios of the upper 15 cm of mineral soil will be lowest. Preliminary analyses indicate that at least 10 earthworm species are present and densities range from 3 to 275 earthworms/m² across the site. The eastern plots (N=16) have significantly higher earthworm densities than the western plots (N = 18) (p=0.001), suggesting that the earthworms are spreading westward through the site from plots nearest the river. Differences in earthworm densities appear to be driven by a significantly higher abundance of grey bodied earthworms (p=0.000), primarily Aporroctodea spp., in the eastern plots. Lumbricus species are equally distributed in eastern, 57 individuals/m², and western plots, 39 individuals/m².  Earthworm abundance is highest in plots dominated by Fraxinus or Tilia, as opposed to Acer and is correlated with decomposition rates (p=0.0084) and soil moisture (p=0.0288). Ongoing analyses are investigating patterns of earthworm density and composition, forest floor depth, C:N ratios of floor and mineral soil, and leaf-litter calcium.