Researchers are trying to find out why oak species, particularly red oaks, appear to be on the decline throughout much of the central hardwood region, which stretches from southern New England to the Carolinas and Tennessee and west all the way to central Texas.
"Just walking in the woods, we can see that there are a lot fewer oaks than there used to be," said Gary San Julian, a Penn State wildlife resources professor.
At the same time, red maple populations are increasing - but it's not exactly an even trade. Several animal species rely on oaks and acorns as a valuable food source that isn't replaced by maples.
In the wood-products industry, oak and maple often are used in the same products, but oak is much more valuable.
Already, manufacturers are seeing a change in the timber harvest.
"I think what the mills are seeing ... is a lot more red maple in the timber that they're getting," said Paul Lyskava, executive director of the Pennsylvania Forest Products Association. "They're bringing in a lot of stuff that is of lesser value."
Researchers say a number of factors - or perhaps a combination of factors - could be behind the change in forest diversity. Among them:
Although maples have long been part of central hardwood forests, red maples are particularly sensitive to fire. Few red maple trees survived fires in the understory - bushes and small trees that are under the forest canopy - that once moved through the forests every few years. Oaks, on the other hand, used fire as part of their natural regeneration.
A century ago, Cawley said, "you had soils that were many, many, many feet deep, full of organics and full of moss and all sorts of things. When we clear cut 100 years ago, that stuff got up and walked away. We've got less of a soil bank than we've ever had before."
The remaining soil has been polluted by acid rain, stripping important elements from the soil, said Bill Sharpe, a professor of forest hydrology at Penn State University.
Both of these factors favor maples over oaks, the researchers say, because maples are more tolerant of acidic soils.
"Red maple is a generalist, and it grows in a variety of different environments," Sharpe said. "Often times, the generalists, or those more adaptable plant species, do better in polluted environments."
To some extent, these species realignments are part of a natural system, Cawley said. Many eastern forests once were dominated by chestnut and white oak. Red oak only became a predominant species after a disease ravaged chestnut populations.
"There are natural processes, and the system will always try to recover," Cawley said. "If red maple is the way that's right for the system, then red maple will come in, and it will be a step toward something else. The forest is always evolving."
But if the red oak is to be kept from following the chestnut, Abrams said, forest managers need to begin working to defend existing oaks stands.
Deer management practices are a good first step, he said, but need to be coupled with controlled burns of the understory to inhibit maple growth and encourage oak regeneration.
"And we probably need to engage in some selective logging," Abrams said. "If we don't, 100 years from now people are going to be saying, 'Where are all these oak forests?'"
By Dan Lewerenz