FOR years, researchers have decried the impact of acid rain on forests and lakes in the Northeast. The United States enacted new laws five years ago to clean up the atmosphere in hopes of curbing the problem.
Now fresh evidence is emerging that suggests the damage to trees and waterways may be more serious than thought - with important implications for forest diversity, marine life, and animal food supplies.
By probing everything from the chemistry of Adirondack lakes to the health of yellow locust trees in West Virginia, researchers and backyard activists are compiling what they say is a disturbing new portrait of the depth of the decline across the region.
''The forest is going to be dramatically changed,'' warns Orie Loucks, professor of botany and zoology at Miami University in Oxford, Ohio. ''In general, we may see a green layer and maybe a dense green layer - but it will be made up of weedy trees and acid-tolerant shrubs.''
Dr. Loucks has long criticized federal researchers for downplaying the effects of pollution on forests. But in the last few weeks, some of these researchers have also begun to raise red flags.
''I hate to sound alarmist. But looking at the cold, hard facts, if the system operates as our research indicates, then you'll see lower water quality and forest productivity,'' says Walter Shortle, a plant pathologist at the US Forest Service's Northeastern Forest Experiment Station in Durham, N.H.
So far, no one has proved that acid rain is directly responsible for tree deaths in the Northeast. The decline only appears in patches of forest and often affects only certain species. Many forests in the region appear healthy.
What is disturbing is that the swatches of mortality are broadly distributed, are occurring at the same time, and are affecting a broad range of species. For example:
* Activists in West Virginia they have set up survey plots to document the change. In parts of the forest, the canopy has thinned. ''It changes the entire face of the forest,'' says Joe Aliff, a former coal miner and self-taught naturalist. From his backyard, he points out seven trees - including yellow locust, oak, and maple - that are clearly dead or dying. A decade ago, he says, only one would have been visible.
* Concerned about dying red-spruce and fir trees in New England, Dr. Shortle and a team of researchers have been looking at the impact of acid rain on soil. Writing in yesterday's issue of the respected weekly Nature, they claim to have identified how increasing soil acidity depletes calcium in Northeast soils. Calcium is essential for forming wood and is the fifth most abundant element in trees. The results, they say, do not bode well for large swaths of the region's spruce and fir forests or for efforts to reduce acid levels in area lakes, streams, and ponds.
* Late last month, the US Environmental Protection Agency (EPA) released a report on acid-rain levels in New York's Adirondack area. Using the EPA's figures, environmentalists calculate that the current provisions of the Clean Air Act won't stem the tide of acid-rain damage. By 2040, according to the most likely scenario in the EPA report, 43 percent of the 700 lakes under study would be too acidic to support most life.
''There is less [acid rain] and there will continue to be less, but the reductions won't be large enough to stop the grim slide that we've been on for the last 40 years,'' says John Sheehan, spokesman for the Adirondack Council, a not-for-profit environmental research and education group, based in Elizabethtown, N.Y.
Other researchers aren't willing to peg acid rain as the main problem. Forests are such complex ecosystems that many say it is too early to raise the alarm over acid rain. A combination of factors - diseases, insects, hard freezes - may have had as much or more of a role in forest decline than acid rain, they point out.
In Warren, Penn., for example, a team at the Northeastern Forest Experiment Station is trying to understand why sugar maples in the northern part of the state have not reproduced large stands of new maples since at least the 1950s. Now the old maples are dying and they don't know why.
One factor behind the regeneration problem is an exploding deer population, which has eaten many of the sugar-maple seedlings. ''But there are clearly other things going on,'' says Susan Stout, project leader of the research team at Warren. In all likelihood, ''it's going to be interactions'' that explain the decline of the sugar maple, ''not a single smoking gun,'' she adds.
The scientists' latest clue comes from the soil. They have found that sugar maples do badly in soils with deficient amounts of calcium and magnesium. In other areas of the state where these minerals are more abundant, sugar maples remain quite healthy - even in areas where acid rain is more prevalent. Furthermore, when researchers here applied dolomitic limestone on sugar maples, the old trees recovered significantly after five years. The limestone is rich in needed calcium and magnesium.
Yesterday's Nature article by Shortle and colleagues Gregory Lawrence of the US Geological Survey and Mark David of the University of Illinois at Urbana-Champaign, provides fresh clues as to acid rain's role.
Until now, the dominant explanations for calcium loss in soils were a net increase in forest growth, which would boost demand for calcium, or acid rain acting directly on the organic layer. Because roots give off hydrogen in exchange for the calcium, the researchers say, one might expect to see the concentration of hydrogen increase as calcium decreased, if forest growth was responsible.
If acid rain were the main culprit, one might expect to see the calcium fall as soil acidity rose. Yet neither of these relationships appeared. Instead, the decline in calcium was accompanied by a marked increase in aluminum.
Aluminum occurs naturally in the mineral layer of forest soils. But when released by acid rain, it spreads upward into the organic layer, where plant germination and rooting occur. The aluminum presents the tree and its environs with a double problem. Past research has indicated that when aluminum is absorbed by tree roots, it blocks the roots' ability to absorb nutrients, including calcium.
But the new work suggests that aluminum also displaces the calcium on soil particles, freeing it to be washed away. This not only reduces the calcium available for plants, but reduces the soil's ability to act as a buffer for the acid rain as it seeps to the mineral layer.
''In calcium-poor sites, you'll begin to see a die-off in the older trees at a higher rate,'' Dr. Lawrence says. ''Small trees will come up, but they will grow in nutrient-impoverished conditions.''
Other factors are also at work. The research team points to nitrogen-dioxide emissions, which have not been significantly reduced under the Clean Air Act. Loucks at the University of Ohio points to the effects of ozone as another contributing factor.
''It may be premature to think that we know the causal agents,'' he says. But ''those forests have for as long as we've known them had insect outbreaks and had drought and disease. And yet when we first came to them were older than these forests today and were not characterized by insect outbreaks that caused death.'' Now, in many parts of the Northeast, that has changed.