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Baraboo Hills Study Shows That Deer Aren’t Depleting Woodland Plant Species

Next time you admire the plant diversity in a southern Wisconsin woodland, thank the neighboring farmers. A University of Wisconsin-Madison study has shown that while deer numbers in parts of south-central Wisconsin were above most damage-threshold estimates, foraging whitetails did little damage to woodland plants because they filled up on farm crops.

Biologists call whitetails “keystone herbivores” – they”re the biggest eaters in the woods. Too many hungry deer can not only damage plant communities, but the changes in vegetation can lead to cascading effects on other members of the woodland community, including songbirds and small mammals.

Wildlife ecologists at UW-Madison”s College of Agricultural and Life Sciences studied eight areas in the Baraboo Hills, in deer management units 54B, 70 and 70B. The Baraboo range contains the largest contiguous tract of southern upland forest and more than half of the native plant species in Wisconsin. Many of the plants are considered “rare” in the state.

Graduate student Rebecca Christoffel studied both large (500 acres or more) and smaller (100 acres or less) sites, all with nearby croplands. Deer numbers at the sites were estimated at 29 to 47 per square mile; Department of Natural Resources overwinter goals for the management units were 26 to 36 deer per square mile. In general, deer densities were 30 percent to 40 percent above overwinter goals during winter 1996, and at or just above overwinter goals by winter 1997.

To measure the effects of deer browsing at each site, Christoffel built fenced exclosure plots, which kept out deer but let in rabbits and other plant eaters. Over two years, she monitored browsing rates and changes in plant density and reproductive rates in the exclosures and nearby unfenced plots. She sampled the vegetation to document species richness and diversity and determine browse preference ratings for selected plants.

Trees on the sites included red oak, white oak, big-toothed aspen and red maple. The shrub understory consisted of maple-leaved viburnum, nannyberry, gray dogwood, blackberry, black/red raspberry, red maple and sugar maple seedlings, hop hornbeam, and American hazelnut. Herbs on the sites included mayapple, wild geranium, violets, large-flowered bellwort, American germander, Solomon”s seal, and enchanter”s nightshade.

“The overwinter deer density estimates in the management units in which our sites were located were equal to or higher than previously published damage-threshold estimates,” Christoffel says. “However, despite these density estimates, no herbaceous plants that we sampled were browsed more than expected given their availability, and most plants had browse rates under 10 percent. The only evidence for damage that we encountered were the reports on deer damage to adjacent agricultural crops. The DNR overwinter deer density goals and associated harvest quotas appear to be adequately protecting oak forests in this area.

“Nothing that we observed or measured suggested that deer impacts on native communities warranted a management response,” she says. “The threshold for deer damage to native plant communities in this agricultural landscape may be much higher than in heavily forested, nonagricultural landscapes. No plants were heavily browsed in oak woods – there are just too many crop fields surrounding these patches for deer to spend their time selectively foraging in oak forests. Crop fields provide a concentrated, nutritious forage base and I believe they are really buffering the surrounding native plant communities.

“Had we sampled exclusively along deer trails in oak forests, I might draw a different conclusion. Where I did find significant browse, it was along established deer trails or very near them,” Christoffel notes. “It would be very interesting to compare browse preference ratings for the same 21 herbaceous plant species that were examined in this study with browse preference ratings in an unhunted deer population, or in an extensively forested area without the buffering element of agricultural crops nearby.”

(In a study of floodplain forests along the Wisconsin River, Christoffel found browse rates under 25 percent on hunted properties. On unhunted property, browse rates sometimes exceeded 80 percent on preferred plants.)

“Given the results of long-term studies in parks and other non-hunting areas, I would advise landowners to actively participate in deer management. If deer populations exceed DNR goals, the potential for plant damage increases,” Christoffel notes.

If deer densities increased substantially in these oak woods, you would have more deer trails traversing the woods and eventually deer would hurt the plant community through browsing and trampling, Christoffel says. They would also add nitrogen to the soil through their waste products, and by trampling they would be making the soil more hospitable for other plant species. “However, these would primarily be weedy alien species that we would not be very happy about seeing an increase in,” she says. “Deer also disperse seeds of garlic mustard, which is a horrible alien pest species that is really going gang-busters in our woods. With an increase in deer numbers, I believe that this and other exotic species might be distributed more rapidly across the state.”

Oak/hickory forests cover more than 22 million acres in Wisconsin, accounting for 40 percent of the timber harvested in the region. More than half of Wisconsin”s commercial forests are under non-industrial private ownership.