Linkage between exotic earthworms, understory vegetation and soil properties in sugar maple forests

•The effects of earthworms on forest plants and soil communities were quantified.•Earthworms influence forest floor thickness and soil microbial community, and their presence is also correlated with soil pH.•Plant diversity decreased in the presence of earthworms.•The abundance of some plant species was negatively affected by earthworms.•Earthworm invasion of sugar maple forests threatens the biodiversity and tree recruitment in this ecosystem. The comminuting and soil mixing activities of earthworms can affect soil physical, chemical and biological properties, which in turn can influence plant growth and survival. Accordingly, there is growing concern that the spread of exotic earthworms into northern temperate forests may compromise biodiversity and tree species recruitment. We report on a study where we sampled earthworms, soils, and understory plants in plots established in 40 mature sugar maple stands distributed over 3 areas in the Eastern Townships of Southern Québec (Canada). Earthworms were found in 19 of 40 plots, and earthworm frequency of occurrence (Efo) as well as the complexity of earthworm communities reflected human accessibility to the plots. Plant species richness decreased, and species evenness increased, with Efo. The Efo was related to a decrease in the cover of 5 plant species, and to an increase in the cover of 2 other plant species or plant functional groups. Increasing Efo also correlated with higher soil pH, lower forest floor thickness and lower soil C:N ratio. Among these 3 variables, redundancy analysis (RDA) revealed that soil pH and forest floor thickness correlated with plant community composition. Based on neutral lipid and phospholipid fatty acid profiles, we found that soil bacteria and fungi increased with a decrease in forest floor thickness, bacteria and arbuscular mycorrhizal fungi (AMF) increased with soil pH, whereas actinobacteria and AMF increased with Efo. We discuss the possible mechanisms by which earthworms might directly or indirectly alter understory plant community composition. By considering the location and land use management of each study site, our study provides further evidence that the spread of exotic earthworms in sugar maple stands of Southern Québec may be linked to human activities, with implications for further research and conservation issues.