Deer and earthworms modify forest responses to climate change

Background/Question/Methods

Global change agents, including rising temperatures, overabundant herbivores, and nonnative invasive species are altering ecosystem processes, structure, and composition. The presence and relative strengths of these agents will determine the pace and direction of ecosystem change. For example, warming temperatures are predicted to cause temperate tree species to expand north into boreal dominated forests, while browsing by white-tailed deer may hinder the expansion of palatable temperate species. Changes in the soil environment, due to nonnative earthworms, may further change understory plant communities, resulting in novel, depauperate plant communities. In two separate studies using field data collected at 139 sites, we examined the influence of these global change agents on the understory of mixed temperate-boreal forests across the upper Great Lakes region in central North America. 

Results/Conclusions

Study results indicate that climate, deer, and earthworms have interrelated roles in northern forests. Growth rates of temperate and boreal saplings were influenced by summer temperature and deer browse pressure. The potential magnitude of growth reduction due to heavy browse pressure and limiting temperatures was similar, up to 50%, and the relative performance of competing species shifted depending on the levels of these drivers. Positive temperate sapling growth responses to temperature were eliminated by heavy browse pressure, tilting growth rates in favor of less palatable boreal conifers at all but the warmest sites.

Evidence of nonnative earthworm presence and disturbance was found at 93% of our sites. Through structural equation modeling, we found that earthworm disturbance was positively related to precipitation and soil pH while deer density was correlated with temperature. Increasing earthworm disturbance and deer density were associated with simple understory plant communities dominated by graminoid cover. These open understory communities were in turn positively related to increased browse damage to maple (Acer spp.) saplings. Earthworms and deer appear to be modifying forest responses to warming temperatures and hindering the expansion of some temperate tree species.