Sharpe is concerned that acid rain may be harming the northern red oak. Red oak regeneration is poor in many parts of the state. The abundance of hay-scented fern, an acid-loving species, prevents tree seedlings from growing.

Sharpe suspects that acid rain is the primary reason why seedlings fail to become established on sites where large sugar maple trees have died. To test this hypothesis, he planted seedlings in soils at declining sites. He fenced out the deer and weeded out the ferns, but the seedlings still didn’t grow. In a greenhouse experiment, he learned that sugar maple seedlings won’t grow in soils containing less than a certain ratio of calcium to aluminum, and almost all of the seedlings died. “These soils simply can’t support the sugar maple anymore,” he says.

But soils aren’t the same all over the Allegheny Plateau, explains Horsley. They’ve had different histories over the past half million years. Some areas were glaciated. When the glacier receded, it scraped away the upper crust, exposing fresh surfaces to weathering. Glaciated soils are thus younger and more nutrient- rich, and the healthiest sugar maple stands grow on glaciated soils. Healthy stands also can be found on lower slopes of unglaciated ridges, where nutrients have washed down from the upper slopes.

Identifying soils that are most vulnerable could help foresters know where to concentrate their management efforts for insects and disease. “Foresters can look at geologic maps and locate the potentially vulnerable stands, but these maps aren’t necessarily going to give you fine-tuned information,” Horsley says. “Now, we’re looking for site-specific indicators that will help foresters know where good nutrition can be found.”

Because herbaceous plants have shallow roots, they are sensitive to nutrition. Some species, like the maidenhair fern, only grow on sites with lots of calcium and magnesium. “If you see maidenhair ferns at a site, you can figure the nutrition is likely to be good there for the sugar maple,” he says. Other potential bioindicators include wild leeks, sharp-lobed hepatica, and blue cohosh. “Once we have a better sense of nutrition at a finer level, we can make a more informed decision about where to spend money to suppress insects.”

Sharpe favors the use of soil testing to determine what areas of the forest are suffering from acidity and aluminum toxicity. He and graduate student Jonathon Lyon refined a simple soil test to determine the ratio of plant-available calcium to aluminum. Ratios less than one, Sharpe and Lyon believe, indicate problems for sugar maples, northern red oak, and other sensitive forest trees. Sharpe worked with staff at the college’s Agricultural Analytical Services Laboratory to offer this “aluminum stress test” to the public. For a modest fee, landowners and forest managers can get their soil tested to determine whether or not aluminum is a problem.

Bill Sharpe believes that as soils continue to acidify, more trees than just sugar maple will begin to decline.

“In Pennsylvania, we need a routine monitoring program, where we can look at components of our ecosystem that can serve as early warning signs of environmental degradation,” say forest hydrologist Jim Lynch, who runs the Pennsylvania Atmospheric Deposition Monitoring Network (see “A Brief History of Acid Rain”). Lynch would like to see scientists routinely monitor the headwater streams that drain forested watersheds. “These watersheds integrate all of the environmental impacts occurring on them and manifest these impacts in the conditions and biota of the streamwater,” he explains. Like taking a blood test, if researchers know the chemical composition of these streams and the macro-invertebrate population—or some other index to measure biodiversity or health—they can go back, over time, and determine if the environment has gotten better or worse.

“We need to look at a lot of things, such as the effects of other air contaminants,” Lynch says. Some of our lakes and streams in Pennsylvania have mercury fishing advisories. The amount of mercury in rainwater is five times the drinking water standards. Half of the mercury in the Great Lakes probably comes from coal and waste incineration. These metals accumulate in nature. If I eat fish with mercury, for instance, that mercury’s going to be deposited in my body and stay there.

“We’re also starting to look at other trace metals in the atmosphere, like selenium, arsenic, and lead. We need to look at organic compounds as well. We know very little about the fate and distribution of organic compounds released to the atmosphere.” For example, a recent study by the U. S. Geological Survey found high amounts of triazine and atrazine in rainwater in the spring. These herbicides are used extensively on agricultural fields to control weeds. “They’re falling on everything,” he says, “from forests to sensitive aquatic environments. We don’t have standards for them.”