Interactive effects of deer, earthworms and non-native plants on rare forest plant recruitment

Multiple biotic and abiotic factors, operating at several stages may affect plant demography and recruitment. Across forests in North America increased white-tailed deer (Odocoileus virginianus) abundance, non-native earthworms and non-native plant invasions are likely to generate single and interactive effects. We evaluated effects of these stressors on recruitment of three rare plants (Aristolochia serpentaria, Carex retroflexa and Trillium erectum). We conducted a multiple year seed addition experiment using paired open and fenced plots (experimentally excluding deer) at 12 forested sites that differ in earthworm density and non-native plant cover. We found strong microsite limitations for C. retroflexa, which completely failed to establish after a 3-year period despite successful germination in greenhouse trials. Addition of A. serpentaria and T. erectum seed resulted in successful seedling emergence; however, A. serpentaria recruitment steadily decreased over the study period. We found no significant effects of non-native Alliaria petiolata or Berberis thunbergii on seedling recruitment but surprisingly, we found strong positive effects of non-native Microstegium vimineum on A. serpentaria and T. erectum. Deer exclusion resulted in increased T. erectum emergence and seedling survival. Earthworm abundance negatively affected survival of T. erectum seedlings, and the effect manifested itself through interactive effects with fencing and non-native plants. Comparing recruitment through seeding addition to recruitment through transplants in a parallel study, we identified strong potential for T. erectum restoration using seed addition, but this will be dependent upon a significant reduction of deer impacts. Restoration of A. serpentaria and C. retroflexa will require manipulation of microsite conditions to promote recruitment.