Research: Why aren’t forests regenerating?

University Park, Pa. —Researchers in Penn State’s College of Agricultural Sciences over the next several years will study multiple factors affecting forest regeneration in Pennsylvania.

Working collaboratively with the Pennsylvania Game Commission and the state Department of Conservation and Natural Resources, investigators at Penn State have identified four study areas on the Bald Eagle, Rothrock and Susquehannock state forests.

This study will not simply measure the effects of deer browsing on forest vegetation, according to Duane Diefenbach, adjunct professor of wildlife ecology and leader of the U.S. Geological Survey’s Pennsyl­vania Coopera­tive Fish and Wildlife Research Unit at Penn State.

“Rather, we have designed this study to simultaneously address multiple theories about why Pennsylvania forests are having difficulty with regenerating trees and restoring plant diversity,” he said.

During the project – which is expected to last at least five years and perhaps much longer – researchers will closely monitor and adjust deer abundance in the study areas using hunting and the Game Commission’s Deer Management Assistance Program, also known as DMAP.

Under that program, the commission allows landowners, such as DCNR, to request that area-specific doe-hunting permits be sold to hunters to increase the deer harvest on their land. As a result, changes in the number of DMAP permits will occur in units involved in the study.

Researchers will monitor the study areas to document hunter experiences, deer densities and vegetation response to deer and habitat experiments. The objective is to determine the relative importance of deer density, competing vegetation, silvicultural treatments, seed sources and other factors affecting forest tree regeneration and plant species composition.

“We know all these factors influence forest vegetation,” said Diefenbach. “What we want to learn is how these factors interact to influence vegetation changes so that ultimately we can do a better job of managing deer populations.”

For decades, deer and forest management in Pennsylvania has operated primarily on the idea that managing white-tailed deer abundance will influence forest regeneration, according to Marc McDill, associate professor of forest management, who is leading the study with Diefenbach. In large part, this rationale is based on decades of observations of the adverse effects of deer browsing.

“However, only recently has research been conducted using deer enclosures to examine the effect of deer browsing on tree species regeneration and composition,” McDill said.

“A concern has arisen because indicators of deer browsing have declined in some management units, but regeneration is not improving – or not improving as quickly as expected.”

As a result, McDill explained, the relationship between deer browsing and forest regeneration remains unclear.

“The research is designed to reduce the uncertainty regarding how to interpret results of the monitoring programs currently used by the Game Commission and Bureau of Forestry to make deer-management decisions.”

On these study areas, some deer have been marked with radio collars and ear tags. All marked deer are legal for hunters to harvest provided they have the appropriate license or permit. If a hunter harvests one of these marked deer, the researchers ask that he or she call the toll-free number on the collar or tag.

Hunters will receive a $100 reward for reporting the harvest of specially ear-tagged deer if the hunter calls the toll-free number on the tag. Rewards are not issued for radio-collared deer.

“Issuing rewards is a cost-effective way for the research project to estimate harvest rates accurately on the study areas,” said Chris Rosenberry, deer and elk section leader for the Pennsylvania Game Commis­sion.

In addition, all deer hunters on the study areas pursuing either antlered or antlerless deer will be required to register with DCNR.

Registration is free and can be done after July 1 through the DCNR website: Also, signs will be posted along the boundaries of all study areas with a toll-free number to call.

An important aspect of this study is to learn more about how hunters use regular wildlife management unit antlerless deer licenses, as well as DMAP tags, to harvest antlerless deer, noted Emily Just, wildlife biologist with DCNR.

“To better understand how DMAP works, we need to know who is hunting on these study areas and learn about their hunting experiences and success,” she said.

“Hunters will be surveyed after the hunting season to determine hunter harvests and success rates.”

According to Diefenbach, the research aims to answer some questions that have proven to be controversial in Pennsylvania.

“In the last decade, deer impacts were reduced in many wildlife management units with lower deer populations,” he said. “Anecdotal observations suggest forest regeneration has improved, but large-scale, quantitative forest analysis and monitoring has failed to indicate major changes in tree regeneration.”

The ability to explain the lack of change in tree regeneration despite deer impact reductions is critical to the Game Commission’s deer-management program, Diefenbach pointed out. “The commission needs more information about how the measure of deer browsing impact is related to deer populations and forest regeneration.”

The Game Commission currently uses a measure of deer browsing intensity developed by the U.S. Forest Service. This study will help assess how sensitive this measure is to changes in deer density.

However, even if the deer browsing measure is not a good indicator, the study is designed to find ways to either improve monitoring or identify better measures of deer browsing impacts, the researchers said.

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