Individual and combined effects of non‐native earthworms and native white‐tailed deer on understorey plant survival, growth and reproduction

Abstract

Forests in northeastern North America have undergone dramatic transformations due to losses and gains of species, changes in land use and pollution. Historic stressors combined with new threats of white‐tailed deer and non‐native earthworms are threatening native plant diversity. We developed a transplant approach to gauge the importance of deer and earthworms in allowing understorey species recovery. This approach, instead of censusing existing individuals, avoids problems imposed by past land use, dispersal limitation or climate change. We selected 20 native species in different taxonomic and functional groups, with different palatability to herbivores, different life‐history strategies and a range of plant traits (%N, specific leaf area [SLA]) and our selection included species indicative of primary or secondary forests. Using a 2 × 2 full factorial design, we planted species into fenced and unfenced plots with and without existing earthworm invasions in five secondary hardwood forests. We measured survival, plant growth and reproduction over 4–6 years. Earthworm biomass was associated with increased survival of 13 and decreased survival of five species. Surviving transplants grew taller (eight of 12 species measured) and wider (seven of nine species measured) in earthworm plots but were more likely to be attacked by insects. Excluding deer benefitted most species' survival and growth. Taxonomic class, SLA or foliar N did not affect species sensitivity to deer or earthworms. Synthesis. Secondary forests in our region continue to provide suitable habitat for native species. Earthworms and deer are major structuring forces affecting survival and growth of our transplant species, but their impacts are species‐specific and change over time. We observed poorly recognized indirect negative impacts of deer on many unpalatable species. Taxonomic class and differences in plant traits offered no important insights regarding vulnerability to deer or earthworms. Transplanting native species into secondary forests can succeed in restoring understorey communities, but will require fencing or substantial deer population reductions to be successful long term.