The September woods were full and green, the acorns just starting to drop. Eight deer jumped away as a van and several trailing cars parked along a gravel road on Laurel Hill in western Pennsylvania. Scientists from Germany, Sweden, and Canada, as well as representatives of the Sierra Club and the U.S. Forest Service, followed Penn State forester Michael Demchik down a grassy path. "Even a family with kids hiking along this trail could see something's definitely wrong with these trees," Demchik said.
He stopped beside a newly dug soil pit, 5 feet deep. The visiting scientists gathered around. One climbed into the pit and began breaking off hunks of golden clay and passing them out. Thunder rumbled and the clouds drew close. The scientists pulled up their hoods, readied their umbrellas. Already that morning they had visited a clear-cut that refused to grow back, ferns and sphagnum bog and bushy striped maple replacing the tall oaks and black cherry trees that had been timbered three years before. They had walked through another cut that, after eight years, had grown up entirely as a black cherry thicket, instead of the mix of species formerly there. Here, against the darkening sky, the snags of dead oaks looked stark and disquieting.
"There's 40 to 60 percent mortality in the overstory here," Demchik began. "The overall growth rate is lower than normal, and there's no response by the understory trees to the thinning of the canopy." Ordinarily, a small tree in the understory would respond with a growth spurt when the death of a large neighbor let the sunlight filter farther through the leaf canopy. Here, almost the reverse happened. "The leaf fall is less," Demchik noted, "and there's significantly lower calcium and potassium in the foliage. The soils of the organic layer are no different, but at deeper levels, at the A and B levels, the soil's different than at a healthy site. Here there's a lower calcium-to-aluminum ratio, which means there are higher levels of aluminum available—"
He thrust his notes under his jacket as the hail came pelting down. He raised his voice, but the noise of hailstones hitting dry leaves was too loud to talk over. Lightning crackled overhead, and the scientists scampered for their cars: A stand of dead and dying trees is no place to be in a thunderstorm.
Demchik, who recently earned his Ph.D. at Penn State, is studying the decline of the red oak forests of western Pennsylvania. With his former adviser, Bill Sharpe, a professor in the School of Forest Resources and a member of the Environmental Resources Research Institute, Demchik is sorting out what's stressing the trees and how forest managers might coax the oaks to come back. His audience in the stormy woods was foresters and soil scientists attending the 1998 Pennsylvania Acidic Deposition Conference.
Laurel Hill, they learned, is part of a 25,000-acre forest at an elevation of 2,600 to 2,900 feet. The forest receives 42 inches of precipitation a year—much of it as snow and fog—and hasn't had a major fire since it was last logged off in the early 1900s. The large pines and hemlocks cut then did not grow back, but were naturally replaced by a mixed stand of red oak and black cherry, along with various maples and other minor species. Since then, too, white-tailed deer were reintroduced; the deer, brought from Michigan in 1909, are currently at numbers above the carrying capacity of the woods.
"We should be logging 150 to 200 acres a year," said Dave Williams, the district forester for the U.S. Forest Service, "following the hundred-year cycle recommended by the Forest Service. But we've been holding off. We don't have re-generation already established on the ground"—by which he means there are no oak saplings in the understory ready to replace the tall trees cut. "We've tried a number of schemes. Shelter wood"—a forestry technique that leaves a belt of trees to shelter saplings—"deer fencing, spraying to get rid of the ferns and the striped maple. Even fenced, though, ferns and black cherry will dominate. You might ask, What's wrong with black cherry? It's a valuable hardwood. But the point is not simply to have trees, it's to have a mix of species. We want deer, bear, and turkey in these woods." Without healthy oaks and plentiful acorns, the game will find little to eat.
After three years, a clear-cut should be brushy and thick: those at Laurel Hill were open fernlands. Hay-scented ferns, other researchers have proved, can outcompete oaks, choking off the young seedlings. But weeding out the ferns isn't enough to grow an oak woods. In a series of test plots, Demchik compared different regimens of weeding, fencing, lime, and fertilizer. "Without fencing out the deer," he says, "you get no regeneration at all." Deer love the succulent shoots of stump sprouts and would rather lunch on oak seedlings than on any other forest plant. Yet even with the deer excluded by an electric fence, the trees did not grow at the expected rate. Only when the seedlings were fenced in wire mesh cages, and the soil was weeded, limed, and fertilized, did they respond with vigorous growth.
These results lead Demchik and Sharpe to believe that, in addition to deer, ferns, and other stresses like gypsy moths and occasional drought, the trees are battling air pollution. Laurel Hill is close to Pittsburgh, a city known for years as a smoky and dirty town. The Pennsylvania Turnpike cuts through the forest nearby, and other major highways ring it. Car exhaust and industrial pollution, over the years, have combined with other factors to make the rainfall here highly acidic. Dry particles washed down with the rain have reacted with the calcium, magnesium, and potassium in the soil, leaching them out. "When the available cations of calcium are gone," Demchik explained, "the acid deposition starts leaching aluminum. This gets into the water in the soil and is drawn into the trees through their roots." Aluminum is toxic to trees. Worse, on the poor soil of the ridgetops, where most of the forest lies, calcium was always limited. With the high rate of leaching, the red oaks can't draw enough calcium from the soil to grow. Mature trees die more easily, while seedlings and stump sprouts don't bounce back from repeated deer browsing. Black cherry does slightly better on calcium-poor soils; ferns and striped maple thrive.
One solution is to plant and fence red oak seedlings in areas freed of ferns (by herbicide) and well limed and fertilized. When presenting his plan to the Forest Service representatives, Demchik noted that such intensive forest management could actually be cost-effective, given the high board-foot price of red oak lumber. Another option is to introduce more acid-tolerant species, like chestnut oak or various pines, that provide habitat and food for game animals although their lumber value is less. Or, as the Sierra Club representative suggested, we could attack the problem by reducing air pollution.
Michael Demchik, Ph.D., is a postdoctoral fellow in the School of Forest Resources, College of Agricultural Sciences, and the Environmental Resources Research Institute, Land & Water Research Bldg., University Park, PA 16802; 814-863-0130; mcd135@psu.edu. William E. Sharpe, Ph.D., is professor in the School of Forest Resources and the Environmental Resources Research Institute; 104 Land & Water Research Bldg.; 863-8564; wes@psu.edu. Their work at Laurel Hill is funded by the Allegheny Foundation.