Impacts of Deer and Earthworms on Understory Forest Plant

Northeastern North American forests experience a myriad of stressors, influencing their capacity to sustain diverse communities, provide ecosystem services and replenish valuable timber resources. Expansions of non-native earthworms and native white-tailed deer populations have occurred simultaneous with other stressors, but conservation and management mandates are focused on individual threats. Here, we provide a mechanistic explanation of the individual and combined effects of deer and earthworms on forest understory plant species using experimental plantings in a 2 x 2 factorial design. We seek to understand what makes many species decline under these altered forest conditions, and to test the viability of restoring plant communities. First, we assessed earthworm impacts on cycling of a broad spectrum of nutrients. We found earthworms are associated with lower soil P, but higher concentrations of other nutrients in the A horizon, including Ca, Mg, K and S. Despite this, we saw little rooting in the A horizon of earthworm invaded plots. This could be due to the stressful rooting conditions in the surface soil of the A horizon created by earthworm activity. If non-native plants that have coevolved with earthworms are able to access these nutrient-rich pools that are largely untapped by the background native vegetation, they may proliferate in earthworm-invaded forests. Of the native species were assessed, successful species were able to incorporate additional Ca into their tissues, and maintained consistent tissue P despite earthworm-associated depletions in soil. Species that declined could not capitalize on higher soil Ca in earthworm invaded plots, and had lower concentrations of P in roots and leaves. For the next two studies, we used transplant experiments of species with a breadth of growth forms to standardize species pools. We explored indirect methods of deer and earthworm impact on fine roots, mycorrhizal associations and soil nutrients. We found earthworms and/or deer decreased % colonization by arbuscular mycorrhizal fungi (AMF) in one species (Quercus) but not others, negatively affected soil nutrient concentrations and pools and increased or decreased the proportion of fine roots relative to total root length. However, this did not reliably translate to changes in seedling survival or biomass. Finally, we followed transplants of 20 native understory species over four to six years. Initially, seedlings of most species had poorer establishment in earthworm invaded plots, but by the end of the experiment, earthworms benefited 13 of 20 species and negatively affected five. Earthworm impacts on seedling survival was largely decoupled from impacts on growth and reproduction, with most species performing better in earthworm-invaded plots. Deer limited most species' growth, reproduction, and (to a lesser degree) survival, including tall, non- palatable species. Survival of species with high foliar nitrogen concentrations were slightly diminished in the presence of both deer and earthworms. Despite lower survival of some species in earthworm-invaded plots, we were successful in establishing shade-intolerant species into forests. This suggests that after initial changes to the forest floor from earthworm invasion have stabilized, restoration of the vegetation can be successful if it is paired with deer management.