PINE GROVE MILLS, Pa. -- Bill Sharpe has no trouble finding a thick stand of red maple on a mountainside south of Pine Grove Mills. Finding a red oak, though, isn't as easy.

Sharpe, a professor of forest hydrology at Penn State University, is one of several researchers trying to determine why oak species, particularly red oaks, appear to be on the decline throughout much of the eastern 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:

* A century of actively suppressing forest fires has made it easier for maples to expand their range, said Marc Abrams, a Penn State professor of forest ecology and physiology.

* Soil quality and quantity have fallen, particularly on mountainsides that once were clear-cut for their abundant chestnut, oak and other hardwoods, according to Jon Cawley, an assistant professor of biology at Roanoke College in Salem, Va.

* In New York, Pennsylvania, Virginia and West Virginia, oaks also are stressed by deer overpopulation. Researchers don't know the reasons, but they do know that deer would rather eat oak than maple.