Anthropogenic factors such as elevated deer populations, invasive earthworms or climate change may alter old-growth forests of the Upper Midwest region of the United States. We examined demographic trends of woody species across all size classes over 35 years in a late-successional forest dominated by hemlock (Tsuga canadensis), sugar maple (Acer saccharum) and yellow birch (Betula alleghaniensis) in Michigan's Upper Peninsula using two sets of permanent plots. For the duration of the study period, species that were less-preferred white-tailed deer (Odocoileus virginianus) forage, especially sugar maple, comprised a much higher fraction of all seedlings and saplings compared to overstory trees. The density of small sugar maple declined across the study period, but no other species became more abundant, creating a more open forest understory. By the most recent census, preferred species for deer browse had been nearly eliminated from the understory, and declines in unpreferred species such as sugar maple were also apparent. We found small changes in temperature (<0.5-1°C rise in minimum and maximum temperatures depending on season) and precipitation (±28. mm depending on season) and little evidence of invasive earthworms impacts. Our results suggest that the sustained elevated deer density is shifting the structure and composition of this old-growth forest. A demographic model showed that if current recruitment, growth and mortality rates were to continue for 500 years the forest would eventually reach a new equilibrium with virtually no hemlock or yellow birch remaining.
Bibliographical noteFunding Information:
We would like to thank M. Billmire, L. Gilner and A. Thrall for their assistance on the 2006 census and M.B. Davis, C. Douglas, K. Walker, S. Hotchkiss, J. Blackett, M. Lindquist, S. Hill and many others for their work on previous censuses of the Davis plots, P. Nelson for his work developing the forest simulation model, and T. Church and R. Evans for their support of field work. M.B. Davis began this study and provided comments that greatly improved this paper. This research was supported by grants from the Mellon Foundation , the Dayton-Wilkie Fund and by National Science Foundation Grants BSR 8615196 to M.B. Davis and J. Pastor, and BSR 8916503 and DEB 9221371 to M.B. Davis.
- Recruitment failure
- Sugar maple
- Upper Michigan
- White-tailed deer
- Yellow birch