Researcher Develops Tool for Measuring Deer Browsing

Duane Diefenbach (right) instructs student Cory Miller (forest science, management) in the proper way to measure young tree growth as part of the rapid habitat-assessment procedure he developed to gauge the amount of deer browsing on large expanses of forest.

A researcher in the College of Agricultural Sciences, working under contract with the Pennsylvania Department of Conservation and Natural Resources (DCNR), recently unveiled a new rapid habitat-assessment tool for state officials to use in measuring the impact of deer browsing on public lands.

Gauging the effects of deer browsing is important because both DCNR and the Pennsylvania Game Commission have expressed concern about the condition of the state’s forests after decades of suspected overbrowsing by too many white-tailed deer. State officials say that desired tree species, such as red oaks, are not regenerating. The Game Commission is changing its deer-management strategy from simply estimating deer numbers to also assessing forest habitat conditions and deer-herd health.

“Measuring deer impacts on relatively small blocks of forestland is not a new concept, with scientists repeatedly making intensive measurements of tree regeneration,” says Duane Diefenbach, adjunct associate professor of wildlife in the School of Forest Resources and assistant unit leader of the Pennsylvania Cooperative Fish and Wildlife Research Unit. “But we are entering uncharted territory here. The question is, can we develop an accurate, cost-effective technique for using these measures across a broad scale to help make management decisions for hundreds of square miles of forest? Except for the Kinzua Quality Deer Cooperative in northwestern Pennsylvania, I am not aware of any study collecting vegetation data directly relevant to deer browsing on such a large scale.”

Diefenbach, his colleagues in the School of Forest Resources, and his most dependable students will be walking transects—with the aid of geographic information system technology—and counting plants. They will tally wildflowers that deer prefer, such as Canada may-flower, jack in the pulpit, Indian cucumber, and trillium. They will count tree seedlings of every species under 3 feet in height, and they will count shrubs and saplings. “We are going to count plant species known to be preferred by deer,” he says. “And we will be quantifying the presence of plants such as mountain laurel and ferns that interfere with the regeneration of trees.

“We have tried to choose simple, quantitative measures—mostly counting plants or recording presence,” Diefenbach adds. “We will be entering the data on field computers that have built-in error-checking routines. Over the course of this summer, we hope to collect data from 3,000 plots over an area of about 500 square miles. The idea is to make this a rapid assessment. We can’t afford to take detailed measurements everywhere. We'd like to spend as little as 10 minutes at a site collecting data and then move on to the next site—ultimately covering as large an area as possible.”

DCNR has directed Diefenbach to assess habitat on 11 tracts enrolled in the Game Commission’s Deer Management Assistance Program (DMAP), which allows hunters to remove deer from specific properties where landowners want to reduce deer populations. DCNR controlled properties entered into the DMAP program are scattered across the state, from the Micheaux State Forest in the southeast to the Delaware State Forest in the northeast to the huge Tioga State Forest in the northcentral to the Gallitzin State Forest in the southwest.

Merlin Benner, a DCNR wildlife biologist based in Wellsboro, believes the rapid habitat-assessment tool being developed by Penn State is important for managing the 2.1 million acres of state forestland. “We have all kinds of protocols for monitoring habitat, but they are pretty intensive and they are intended to look at stands of trees to make timber management decisions on plots from 30 to 100 acres,” he says. “DMAP is on a larger scale, on the order of tens of thousands of acres. We need a habitat-assessment tool that is more applicable across a broader scale and takes less effort.

“The shortcoming of our other monitoring is that it only focuses on tree species,” Benner added. “We [the state Bureau of Forestry] have begun our own browsing surveys using the available literature, but we feel there is the potential for something better, and we hope that is provided by Diefenbach’s work.”

Researchers, such as student Matt Reed (right) (forest science, biology), will use hand-held computer units to tabulate vegetation presence and growth.

Diefenbach knows that deer management and habitat assessment are extremely controversial in Pennsylvania, with many hunters contending that the Game Commission and DCNR have reduced deer numbers to unacceptably low levels on public lands. He also understands that angry sportsmen skeptical about valuing trees over deer are likely to second-guess his methods and doubt his data.

“I am not concerned about that—good habitat is critical for good deer hunting,” he says. “I picked the best students I could find, and our methodology is objective, transparent, and reliable. I am confident about the quality of the data we are going to collect. We are using the best science available and incorporating measures that scientists have proposed as deer-browsing indicators.

“Even if you don’t believe deer overbrowsing is an issue in Pennsylvania forests, at least we are starting to look beyond deer numbers and trying to find ways to assess habitat conditions as they are most relevant to deer,” Diefenbach adds. “At Penn State we have the skills and the tools to provide the best data possible. That’s my objective here.”

The Pennsylvania Game Commission is helping to support the research and is interested in the results, Diefenbach noted. “If this turns out to be a cost effective way to do a quick habitat assessment for the impact of deer on state forest, it will be just as applicable for state game lands, or for that matter for any landowner with large tracts of forest,” he says. “The Game Commission needs tools for assessing habitat on the 1.4 million acres of land it manages, too.”

The Game Commission sees considerable promise in Diefenbach’s research project, according to Christopher Rosenberry, supervisor of the agency’s deermanagement section. “We have started using measures of tree regeneration to characterize forest habitat health by wildlife management unit,” he says. “What is missing is a direct measure of the role of deer browsing on tree regeneration. This research may enable us to not only look at the general forest habitat condition, but also directly measure the impact of deer on forests by wildlife management unit.”

— Jeff Mulhollem