Exclosure Results Implicate Deer in Forest Degradation

Autumn Sabo and Katie Frerker at work

Exclosure Results Implicate Deer in Forest Degradation
Autumn E. Sabo, Ph.D. student in Forestry (aesabo@hotmail.com)

Funding: Friends of Peninsula State Park, WDNR, and UW-Madison Botany Department

After noticing that low cedar foliage and yew shrubs were becoming rare as deer populations grew, folks in Door County wanted to demonstrate how lowering deer numbers would affect forest plants.  About

20 years ago, local citizens partnered with DNR staff to build three deer exclosures at Peninsula State Park.  School kids monitored plants in the exclosures and adjacent outside control areas over several years.  Kathleen Harris, Park Naturalist, counts the number of flowering trillium inside and outside the Nature Center exclosure to track population changes.  In 2011, I sampled two of these exclosures with another graduate student, Katie Frerker, and a team of undergraduates from University of Wisconsin-Madison.  We also visited two other exclosures in Door County, plus 13 more around the region, to learn about the long-term consequences of deer browsing and exclusion.

Sabo and Frerker used fish-eye lens photos, like this one, to estimate the amount of shade cast by forest canopy. This picture was taken near the Nature Center Exclosure.

The obvious change in cedar tree form was an excellent clue that deer were impacting the forest.  Our measurements from the Nature Center exclosure found abundant cedar saplings inside the fence but none outside.  Across the four Door County State Park areas that we sampled, we counted 13 different species in the sapling stage inside exclosures but only three species in outside areas accessible by deer.  Heavy deer pressure appears to be reducing sapling diversity.

Understory plant communities, on the other hand, had similar numbers of species outside the fence compared to inside the fence.  Once we looked beyond just the number of species, we found that the composition differed.  Non-natives and grass-like species were more abundant outside the fence.  Habitat specialists, like rough dogwood and maple-leaved viburnum, frequented the exclosures.  The overall quality of the plant community was lower in areas open to deer.

Frerker stands next to a heavily browsed pine. Note that little foliage exists within easy reach of deer. At the base of the pine, a small spruce is left untouched because it is a less palatable species.

We compared ground flora information from all our 17 exclosure/control areas against another dataset that tracked community changes over 50 years in unfenced Wisconsin forests.  We found that deer are likely responsible for shifting forests across Wisconsin to lower quality, more heavily invaded communities with less vertical structure.  This could spell trouble for the many critters that rely on forest plants, as well as the trees, shrubs, and wildflowers that contribute to the region’s allure.  Exclosures provide evidence that stronger deer control and protection of browse-sensitive species can help reverse, or at least slow, forest quality decline. Thanks to Peninsula State Park for providing these wonderful exclosures that allow researchers and curious park visitors to study interactions between deer and other forest dwellers!