Threats posed by ungulate herbivory to forest structure and plant diversity in the Upper Great Lakes region: With a review of methods to assess those threats

Waller, Donald; Johnson, Sarah; Collins, Rachel; Williams, Evelyn

1. The National Parks of the Upper Great Lakes serve as outstanding examples of natural landscapes, home to plant communities of special significance including late successional forests and wetlands. 2. Large ungulates like deer, elk, and moose are popular and physically impressive wildlife species in the Upper Great Lakes. Deer and moose now act as “keystone herbivores” capable of radically curtailing tree regeneration and the abundance and diversity of forest understory plant communities. 3. Ungulates consume oak and hickory mast, seedlings, and woody browse from a wide array of woody plants. Several conifers including Canada yew, eastern hemlock, and northern white cedar are particularly hard hit by white-tailed deer, though many deciduous trees are also strongly affected including yellow birch, northern red oak, and other species. 4. Ungulates also graze on a wide range of grasses, sedges, and forbs, particularly in late spring and summer when these foods make up the majority of their diet. 5. Ungulate impacts depend not only on ungulate densities but also on context including the abundance of available favored plant species, the nature and rates of their growth, and the availability of alternative food sources. 6. Heavy deer herbivory has also been associated with the invasion and spread of weedy exotic plant species including garlic mustard, buckthorn, and honeysuckle, and possibly the spread of Eurasian earthworms. 7. Because ungulate impacts can be profound and long-lasting, monitoring is important both for anticipating such impacts and for tracking recovery from overbrowsing. 8. Ungulate impacts affect an increasing proportion of landscapes in the upper Midwest. This makes ungulate-free zones scarce and of increasing value for evaluating impacts and providing refuges for those plant species most affected by ungulate browsing. 9. The number, diversity, complexity, and significance of ungulate effects make it important to monitor their impacts. Such monitoring can take many forms and can potentially provide early warning of impending substantial and difficult-to-reverse impacts. 10. Monitoring ungulate impacts on woody plants typically involves either directly tracking stature or the number of browsed twigs or indirect inferences of browsing based on demography. 11. Monitoring direct impacts of ungulates on herbaceous species typically includes tracking visible signs of browsing. 12. It is often of particular value to monitor particular indicator species known to be palatable to ungulates including many rarer species of conservation concern when these are encountered. 13. Given these last two points, we have an incentive to develop synthetic monitoring techniques and methods of analysis that make use of data from several species. 14. Fenced exclosures are of particular value for monitoring ungulate impacts: a) They provide direct evidence of ungulate impacts in a controlled experimental setting. b) Such exclosures can be placed to evaluate ungulate impacts on particular species or in particular areas and to assess how these accumulate over time. c) They can provide visually arresting demonstrations of ungulate impacts with great educational value. 15. It is important to sustain monitoring programs to fully assess the short- and longer-term ecological impacts of ungulate browsing. 16. Monitoring ungulate impacts remains a developing art. Monitoring in the past has often been local and haphazard, focusing on only a few populations or species for a few years. 17. The National Parks in the Lakes States have a special opportunity to design and implement a more comprehensive and sustained monitoring program on ungulate impacts. 18. There may be opportunities to partner with state and other federal agencies in the region to develop and implement such a comprehensive and sustained ‘adaptive monitoring’ program.

Threats posed by ungulate herbivory to forest structure and plant diversity in the Upper Great Lakes region: With a review of methods to assess those threats

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