After more than a decade of closely monitoring regeneration of oak trees on forest tracts around Pennsylvania, researchers in Penn State's College of Agricultural Sciences have begun to understand why stands of the state's most important tree are not replacing themselves after they are harvested.
Amid a raging debate between hunters angry about deer numbers being reduced by the Pennsylvania Game Commission to spur oak regeneration and state Department of Conservation and Natural Resources officials -- who claim too many deer have over-browsed forest habitat -- scientists in Penn State's School of Forest Resources have quietly been measuring the growth of young oaks for 11 years.
Studies conducted across the state by Penn State and the U.S. Forest Service have found that fully 50 percent of the stands studied don't have the regeneration to replace themselves, according to Jim Finley, professor of forest resources. "So where the canopy has been disturbed -- where there has been cutting and there should be regeneration on the ground because there is sufficient light -- half the time it is not happening," he says. "And that's a major problem."
In 1995, professor of forest biology Kim Steiner and colleagues
began a study of stand development on state forest lands within the central third of the commonwealth. "The smallest research tract is 13 acres, the largest is 225 acres," he says. "Some tracts are fenced to exclude deer and some are not, and they differ in a few other important ways. But all are harvested soon after we begin measuring, so the study spans both the old and the new developing stands.
"We are currently monitoring the development of 70 forest stands and our data begin with conditions of the previous mature stand and continue at three-year intervals with characteristics of the new developing stand," adds Steiner. "All of our several thousand plots are permanently marked and can be relocated exactly with global positioning
system instruments."
Finley explains that the inventory conducted by the U.S. Forest Service in Pennsylvania is showing a species shift from northern red oak, which had been the most common species, to other species that are less valuable both commercially and ecologically. Red oak was number one -- now it is 10th and red maple is first. Black birch used to be the sixth most common species; now it is second. Black cherry is now number three. "What is important is that neither maple nor birch nor cherry are preferred browse by deer," Finley says.
Finley maintains that decades of browsing by an overpopulation of white-tailed deer has played a role in what is happening in the forests across the state. "I don't think deer overpopulation is the sole reason, but it certainly contributes greatly," he says. "We've found if we put up fences, we can affect the shift. We can put up a fence to keep deer out, and after we get the forest headed in the right direction, we can build the deer population back up. We believe if you take the deer numbers down, oaks and other valuable trees will begin to come back."
Perhaps the most surprising conclusion to be drawn from the research, according to Steiner, is that conditions at the time of harvest play a very large role in the development of the future stand. To achieve reliable regeneration, fencing of oak stands to keep deer out should occur before harvest. "We have seen repeatedly that doing a shelterwood cut (when some oaks are left to provide seed for regeneration) is not a reliable substitute for a good population of seedlings already present when the stand is cut," he explained.
"Shelterwoods can succeed if a heavy seed crop occurs in the first year, but heavy seed crops are unpredictable. Seedlings that germinate in later years do not survive well under competition from established plants that got there first.
"But we have also disproved a widespread notion that oak seedlings must be large at the time of overstory harvest in order to succeed in the next stand. Quantity can make up for size because the forest environment is not homogenous. Some small seedlings start out in advantageous conditions and can survive the early battle for space. That seems obvious, now that we know it, but our findings go against conventional wisdom and practice."
Steiner and Finley believe the most important thing to come from their work with DCNR is an oak-stocking guide that accurately predicts regeneration. "Before foresters harvest a stand of oaks, they want to know if the stand can regenerate itself successfully," Steiner says. "By supplementing our data set with some archival data that DCNR had collected over a period of decades, we were able to develop mathematical models that predict the contributions that both seedlings and stump sprouts will make to future stand stocking."
The oak-regeneration guide is a powerful tool, according to Finley. "We can look at a stand and say with some confidence that if we cut today, here's what we will see 30 years in the future," he says. "We can predict with some certainty when it is necessary to fence and when it is not because we can tell from what is on the ground whether the deer pressure is light or heavy."
It may seem counterintuitive to erect a fence before tree cutting, but that's what is needed for adequate oak regeneration in Pennsylvania, contends Steiner. "Probably the most important thing is that we are beginning to understand the critical importance of keeping deer pressure low for several years in advance of harvest," he says. "We aren't sure yet how far ahead, but we believe it's something on the order of five years or more.Penn State Ag Sciences News
