Long-lived perennial plants are exposed to novel stressors acting at different temporal and spatial scales. Novel stressors can include nonnative species, as well as native species whose behavior or population size has changed. By assessing survival, growth, reproduction and mutualisms across multiple years, we modeled the response of a suite of species to invasive earthworms and native white-tailed deer. We expected earthworms to negatively affect seedling establishment in early years due to reduction of humus layers and soil moisture buffering capacity in colonized soil. We did not expect deer to influence very young seedlings due to their small size. After establishment, we expected most species to be affected by limited access to mycorrhizal symbionts, and for amycorrhizal species to flourish. Once seedlings grew into the ~10cm ‘molar zone’, we anticipated suppression of palatable species by deer; particularly tall, flowering individuals. We tested the individual and combined effects of deer and earthworms using a full factorial experiment planted with twenty-one native species, repeated at five forested sites.
Results/Conclusions
Initially, seedling survival was almost universally lower in earthworm-invaded plots, with the exception of two fern species. However, a different pattern emerged after three years with improved survival of thirteen graminoids, trees, herbs and ferns in earthworm-invaded plots, and only three herbaceous monocots surviving best in earthworm-free plots. Deer exclusion effects became apparent in the third year, with increased growth and/or flowering in four herbaceous species. Interestingly, deer exclusion increased both the survival and height of Polystichum acrostichoides, an evergreen fern that is used by deer as winter forage. In general, species that eventually benefited from earthworm-invaded conditions had more active mycorrhizal structures, while species whose survival was restricted by earthworms had the opposite pattern. By following seedling growth for multiple years, a nuanced picture of the net effect of deer and earthworm-mediated changes emerges: some species become ‘losers’, others ‘winners’.