Changes in Hardwood Forest Understory Plant Communities in Response to European Earthworm Invasions

Abstract

European earthworms are colonizing earthworm-free northern hardwood forests across North America. Leading edges of earthworm invasion provide an opportunity to investigate the response of understory plant communities to earthworm invasion and whether the species composition of the earthworm community influences that response. Four sugar maple-dominated forest sites with active earthworm invasions were identified in the Chippewa National Forest in north central Minnesota, USA. In each site, we established a 30 x 150 m sample grid that spanned a visible leading edge of earthworm invasion and sampled earthworm populations and understory vegetation over four years. Across leading edges of earthworm invasion, increasing total earthworm biomass was associated with decreasing diversity and abundance of herbaceous plants in two of four study sites, and the abundance and density of tree seedlings decreased in three of four study sites. Sample points with the most diverse earthworm species assemblage, independent of biomass, had the lowest plant diversity. Changes in understory plant community composition were most affected by increasing biomass of the earthworm species Lumbricus rubellus. Where L. rubellus was absent there was a diverse community of native herbaceous plants, but where L. rubellus biomass reached its maximum, the herbaceous-plant community was dominated by Carex pensylvanica and Arisaema triphyllum and, in some cases, was completely absent. Evidence from these forest sites suggests that earthworm invasion can lead to dramatic changes in the understory community and that the nature of these changes is influenced by the species composition of the invading earthworm community.

Invasions of non-native earthworms related to population declines of ground-nesting songbirds across a regional extent in northern hardwood forests of North America

“…Third, forests with heavy Lumbricus invasions are perceived by Ovenbirds to be sub-optimal habitat and are avoided in favor of less impacted areas. In agreement with this hypothesis, Ovenbirds are known to select territories and nest sites with deep leaf litter (Van Horn and Donovan 1994;Mattsson and Niemi 2006) and at least some herbaceous ground cover and shrubs for nest concealment (James 1971;Smith and Shugart 1987), conditions that are characteristic of Lumbricus-free and lightly invaded forests (Hale et al 2006;Holdsworth et al 2007a;Nuzzo et al 2009). All three hypothesized mechanisms could mediate Lumbricus-associated declines; however, further research is required to determine the importance of each.…”

“…All of the following analyses were repeated for: (1) all forest types combined (n = 124), and (2) points in sugar maple and sugar maple/basswood forests (hereafter; maple-basswood, n = 53), because earthworm effects in the study region have been documented to be most substantial in these forest types (Hale et al 2006;Holdsworth et al 2007a, b), and a previous study found significant associations between Lumbricus and songbirds in these forest types (Loss and Blair 2011). We used independent samples t-tests to compare density of each bird species, as well as litter depth, ground cover, and sedge cover, between Lumbricusfree and invaded sites.…”

“…Within a 10 m radius, we estimated shrub density by counting the number of woody plants with diameter at breast height \2.5 cm. Immediately before bird surveys in 2009 and 2010, we measured litter depth and cover of sedges and grasses, variables that have been found to significantly decrease and increase, respectively, with Lumbricus invasions (Hale et al 2006;Holdsworth et al 2007a), and that have been related to Ovenbird nesting success (Loss and Blair 2011). We based average litter depth on measurements from two random locations within 10 m of the point and estimated by pushing a metal skewer through the litter until meeting resistance from rock or mineral soil.…”

“…Invasive earthworms alter soil structure and consume the leaf litter layer (Alban and Barry 1994;Hale et al 2005). These changes affect seedbed conditions, which reduces richness and cover of herbaceous plants, increases dominance by sedges and grasses (Hale et al 2006;Holdsworth et al 2007a), and reduces tree regeneration and growth (Lawrence et al 2003;Larson et al 2010).…”

“…We sought to determine whether earthworms pose a regional threat to ground-nesting forest songbirds by: (1) comparing densities of four species-the Ovenbird, Hermit Thrush, Veery (Catharus fuscescens), and Black-and-white Warbler (Mniotilta varia)-between points invaded by Lumbricus spp. and points that were earthworm-free or only contained Dendrobaena octaedra, a species with minimal adverse effects in the region (Hale et al 2006(Hale et al , 2008; and (2) clarifying the contribution of Lumbricus relative to vegetation structure and landscape Landscape Ecol patterns of forest cover for predicting density of the Ovenbird.…”