Hardwood forests in eastern North America are being colonized by multiple nonnative plant and animal species. Colonization rates can be affected by stand structure and distance from edge. We sampled earthworm densities and understory plant species cover in transects located in paired old- and second-growth forests in Indiana. Two 100-m transects were established within each forest stand during late April to early May in each year. One transect was placed parallel to and within 5 m of a south- or west-facing edge. The second transect was placed parallel to the first. but at no less than 100 m from any edge. Nonnative earthworms and plants were found in forest edge and interior regardless of structural stage (second-growth vs. old-growth). The number of native plant species decreased linearly as the densities of adult Lumbricus and Aporrectodea earthworms and the percent cover of multiflora rose (an invasive plant species) increased. Densities of L. terrestris and Aporrectodea earthworms and percent cover of multiflora rose cumulatively explained 39% of the variation in the number of native plant species found in transects across the state. However, multivariate analyses suggested that the species composition of Indiana understory plant communities was affected more by geography than by earthworm densities. Our results suggest that nonnative earthworms and plants are ubiquitous in Indiana hardwood forests and that they may reduce the number of native plant species.
Nomenclature: Multiflora rose, Rosa multiflora Thunb. ex Murr. ROSMU; nightcrawler, Lumbricus terrestris L.
Management Implications: European earthworms, which are not native to eastern temperate and boreal forests in North America, have been linked to changes in forest soils and to declines in native plant diversity in invaded areas. Earthworm densities appear to be greater under several invasive plant species and researchers have speculated that these earthworms might facilitate plant invasions and vice versa. We sampled six old- and second-growth pairs in Indiana for earthworms and understory plants in 2009 and 2010. The number of native plant species decreased as the densities of adult Lumbricus and Aporrectodea earthworms and the percent cover of multiflora rose (an invasive plant species) increased. We found little evidence that stand structure or distance from edge strongly affected earthworm distributions (with the possible exception of L. rubellus). It seems apparent from this study that nonnative earthworms and plants can establish in the interior of old-growth forest. Consequently, land managers should anticipate changes in nutrient availability, organic matter decomposition, and arbuscular mycorrhizal fungi populations that are associated with earthworms in hardwood forests irrespective of stand age. These changes, which constitute a substantial disturbance to North American hardwood forest ecosystems, seem likely to promote the further colonization of hardwood forests by nonnative plant species. It is unlikely that earthworm invasions can be prevented or reversed and in some areas, such as Indiana, the colonization of forests by introduced earthworms might be largely complete. This study highlights the importance of invasive plant management within the context of multiple plant and animal invaders.