Abstract
Ahistorical drivers such as nonnative invasive earthworms and high deer densities can have substantial impacts on ecosystem processes and plant community composition in temperate and boreal forests of North America. To assess the roles of earthworm disturbance, deer, and environmental fac- tors in the understory, we sampled 125 mixed temperate-boreal forest sites across the western Great Lakes region. We utilized structural equation modeling (SEM) to address the hypothesis that earthworm disturbance to the upper soil horizons and selective herbivory by deer are associated with depauperate understory plant communities dominated by graminoid and nonnative species. Evidence of earthworm activity was found at 93 % of our sites and 49 % had high to very high severity earthworm disturbance. The SEM fit the data well and indicated that widespread nonnative earthworm disturbance and high deer densities had similar magnitudes of impact on understory plant communities and that these impacts were partially mediated by environmental characteristics. One-third of the variation in earth- worm disturbance was explained by soil pH, precip- itation, and litter quality. Deer density and earthworm disturbance both increased graminoid cover while environmental variables showed direct and indirect relationships. For example, the positive relationship between temperature and graminoids was indirect through a positive temperature effect on deer density. This research characterizes an integrated set of key environmental variables driving earthworm distur- bance and deer impacts on the forest understory, facilitating predictions of the locations and severity of future change in northern temperate and boreal forest ecosystems.
Highlights
Nonnative earthworms and overabundant deer have been described as influential ecosystem engineers and keystone species (McShea and Rappole 1992; Hendrix and Bohlen 2002; Bohlen et al 2004)
In northern temperate and boreal forests of North America, these species are associated with a plethora of impacts that lead to significant changes in soil properties and processes and understory plant community composition (Coteet al. 2004; Frelich et al 2006; Eisenhauer et al 2007)
Sites were initially selected for a study of tree regeneration patterns in mixed temperate-boreal forests (Fisichelli et al 2012b) and all sites included a mixture of temperate broadleaf and boreal conifer species in the overstory
Summary
Nonnative earthworms and overabundant deer have been described as influential ecosystem engineers and keystone species (McShea and Rappole 1992; Hendrix and Bohlen 2002; Bohlen et al 2004). In northern temperate and boreal forests of North America, these species are associated with a plethora of impacts that lead to significant changes in soil properties and processes and understory plant community composition Northern temperate and boreal forests in North America developed over most of the past 10,000 years, since the last Pleistocene glaciation, in the absence of large invertebrate decomposers, namely earthworms (Lumbricidae) (James and Hendrix 2004). Changes to the soil environment, for example consumption of leaf litter at the soil surface and incorporation into deeper mineral soil horizons, can be regarded as disturbance with its severity varying with earthworm species identity and density (Hendrix et al 2008; Eisenhauer 2010)
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