Acid Rain Has Not Gone Away

by Paul Donahue
29 August 1997

Back in the 1980's we heard a lot about acid rain. It was a hot topic in the media for awhile, in part because our Canadian neighbors were clamoring for something to be done about the problem. The majority of the pollutants responsible for acid rain were being generated by our mid-western industries and were then drifting across the border to affect Canadian lakes and forests.

Then two events in 1990 allayed most Americans' concerns about acid rain and more or less put an end to the media's coverage of the problem. The first was Congress' passage of the 1990 amendments to the Clean Air Act, which called for reductions in emissions of sulphur dioxide and nitrogen oxide. The second was the publication of the final report of the U.S. National Acid Precipitation Assessment Program (NAPAP).

The federal government spent $500 million on NAPAP, funding numerous studies on the effects of acid rain. The final report of NAPAP, encompassing 27 separate reports, asserts, "There is no evidence of an overall or pervasive decline of forests in the United States and Canada due to acidic deposition or any other stress factor. Moreover, there is no case of forest decline in which acid deposition is known to be a predominant cause."

What the NAPAP report failed to note, however, was the enormous amount of money spent and political pressure applied to dilute its conclusions. Polluting industries had a lot at stake and behaved in the way they usually do when they feel their profits are threatened. There was such tremendous political pressure applied that the final report lacks scientific credibility. It ended up a $500 million whitewash of the problem, to protect polluting industries from having to reduce emissions further. Just the same, the issue has largely slipped from public consciousness.

The health of our trees, however, has continued to decline. The first reports of acid rain damage to trees in the Northeast dealt with forests on the upper slopes of some of the higher mountains of the Adirondacks in New York and the Green Mountains in Vermont. These high peaks were the first to intercept the pollutant-filled rain clouds arriving from the mid-West. Now, however, if you know what to look for, the effects of atmospheric pollutants on our trees can clearly be seen throughout the region.

Trees generally die from the top down. The first visible symptom of decline is usually a thinning of the foliage on the uppermost branches. As the years pass, the tree's crown becomes more and more transparent. Eventually, dead branches can be seen projecting from the crown. The foliage begins thinning out in the middle layers, dead limbs appear, maybe a large limb or two break off in a storm, then the lower foliage begins thinning out, and finally the tree breaks off or blows over in another storm.

Unfortunately, trees die slowly, and this process can take a decade or more. I say this is unfortunate because if trees died suddenly, we might actually be motivated as a society to do something to correct the problem. As it is, the subtle changes now occurring from year to year have not been sufficient to spur us into action. Indeed, they have largely gone unnoticed.

Two species of northeastern trees, Sugar Maple and Green Ash, have been hit especially hard, with virtually every individual exhibiting the symptoms of slow death. Do you remember back when Sugar Maples would first began to turn orange in September? Now the first orange leaves regularly begin to appear on the trees in mid-late August. If a Sugar Maple exhibits more than 15% dieback or more than 55% crown transparency, it can be considered to be in trouble. Some sugarbushes in Vermont and southern Ontario have experienced 80-100% mortality of their Sugar Maples. In southern New England, where Green Ash is more abundant than it is in Maine, many woodlands have been decimated.

Aside from the mountaintops, the worst tree damage in Maine can be seen along the coast, and particularly on exposed headlands and offshore islands. Here the effects of atmospheric pollutants can be seen on virtually every species of woody tree and large shrub. In addition to Sugar Maple and Green Ash, crown-thinning is especially noticeable on Red Spruce, Red, Silver, and Mountain Maples, Paper Birch, Quaking and Bigtooth Aspens, Lombardy Poplar, and Black Locust, with a large percentage of the individuals affected. But the symptoms can also be seen on Black Spruce, Balsam Fir, White Pine, Silver Poplar, Yellow Birch, Black Willow, Red Oak, American Beech, Apple, Pin and Choke Cherry, Horse-Chestnut, Black Locust, Mountain Ash, Bayberry, Staghorn Sumac, Raspberry, and Steeplebush.

Like trees on mountain tops, the trees on exposed headlands and offshore islands are already exposed to heavy environmental stresses. On mountaintops the trees have to deal with strong winds and very cold temperatures while on the outer coast they have to deal with strong winds and salt spray. And while the mountaintops are often enveloped in clouds, the offshore islands are frequently cloaked in fog. The rain in our area is now three times as acidic as it was in pre-industrial times, but our area's abundant fog can be 10 times as acidic as the rain. The trees in both these environments are already living at their limits and any additional stress, such as atmospheric pollutants, can be enough to tip the scales against them.

I began to really pay attention to the problem on an island in Casco Bay off of Harpswell where I have spent much of the fall for the past 22 years. I have photographs taken on the island over that period and can point to considerable tree death that has occurred just in the last six or eight years. If tree death continues there at the same rate, in another twenty or thirty years the island could be devoid of trees. When I visited Eagle Island, a state historic site farther out in Casco Bay, for the first time last year, it seemed like every tree on the island was already either dead or showing signs of damage.

The results of research at the Hubbard Brook Experimental Forest (HBEF) in New Hampshire, published in the 12 April 1996 issue of Science, have disturbing implications for Maine forests in general. Researchers at HBEF found that since 1987 forest growth "has declined unexpectedly to a small rate" due to chemical changes in the soil. The HBEF is very similar to the forest covering most of northern Maine, so it is reasonable to assume that similar declines in growth have occurred here, as well. One of the first measurable effects of pollution damage to trees is a slowing down in growth. This occurs well before there are visible signs of damage, so it might also be reasonable to assume that this decline in growth is just a first step towards widespread forest death. This might seem like a radical statement were it not for the fact that this is exactly what has already happened over large areas of Eastern Europe.

Environmentalists across the state have focused much attention lately on the destructive clearcutting and overcutting of our forests by industry, and rightly so. However, poor management practices are clearly not the only factors affecting the health of Maine's forests, and ultimately may not even be the most important factors. Even if clearcutting, overcutting, the spraying of herbicides and so forth were to be stopped tomorrow, there still exists a distinct possibility that Maine's forests would eventually be destroyed by air pollution.

The latest U.S. Forest Service inventory of Maine forests, published in the summer of 1996, showed that industrial forests were growing not a half cord per acre per year, as they have traditionally, but only a sixth of a cord. This discrepancy has been blamed on industry's management practices, which have undoubtedly played an important role. But how much of this difference is due to poor management practices and how much is due to pollutant-caused slower tree growth? What happens to forest management plans and our forest products industry if predictions of tree-growth do not take into account the slower growth brought on by airborne pollutants?

The acidity of the rain is caused mainly by sulphur dioxide, which comes largely from the burning by power and industrial plants of fossil fuels containing sulphur and from nitrogen oxide, which comes largely from automobile exhaust. The sulphur dioxide reacts in the presence of sunlight to produce sulphuric acid and the nitrogen oxide mixes with moisture to form nitric acid. These acids leach away important mineral nutrients in the soil such as calcium, magnesium, phosphorus and potassium. The acidity also makes toxic aluminum in the soil more available to the trees.

Those things would be bad enough if that was all the trees had to deal with, but the acidity in the rain is just the beginning of the their problems. Ground level ozone, also from car exhausts, damages the chlorophyll in plants and is a primary culprit in forest decline. On top of that there are also airborne toxic metals such as cadmium, zinc, lead, copper, arsenic, barium, chromium, nickel and mercury in the rainfall. When materials containing these heavy metals are incinerated, most of the metals go up the stack and out into the atmosphere. The acidity in the rain makes these metals more soluble, more concentrated and more harmful to the plants, damaging their immune systems.

And then there are substances like pesticides, carried by the wind from aerial spraying operations and dioxins from incinerators. To give some idea of what the trees are dealing with, German researchers have detected some 400 synthetic organic compounds in a single cubic meter sample of air! In actuality, our rain and fog have become a toxic stew, with the pollutants acting synergistically on our trees in ways we are still far from fully understanding. Finally, there is UVB radiation from the thinning of the ozone layer, yet another suspected culprit in forest decline.

One criticism often leveled by opponents of stricter air pollution standards is that acid rain doesn't kill trees, that they are actually killed by cold winters or by insects or fungus or some such agent. In a sense these critics are correct - it is true that cold or insects or fungus or some other pathogen is often the ultimate cause of tree death rather than pollutants. But one might fairly ask why in the latter part of this century are so many pathogens suddenly having such a tremendous effect on so many species of trees. If just one or two species of trees were suffering a decline in eastern North America, maybe it would be acceptable to simply blame the weather or some pathogen and dismiss the problem as unfortunate but unavoidable. But that is hardly the case. Instead, what we are witnessing across most of eastern North America is a pandemic of tree death.

As I mentioned above, Sugar Maple and Green Ash are dying all over New England. Throughout the mid-Atlantic states, extending as far north as southern Massachusetts, and still spreading, Eastern Hemlock has been decimated by the Hemlock Woolly Adelgid, and the same insect has decimated Carolina Hemlocks in the central Appalachians. In Hemlock-rich woods that I knew in Connecticut just ten years ago the trees have been reduced to skeletons. Throughout the same region Flowering Dogwood is suffering widely as the result of anthracnose fungus and Butternut as the result of a canker. Fraser Fir in the Great Smoky Mountains of North Carolina is suffering heavy mortality due to the Balsam Woolly Adelgid. And the list goes on.

We can choose to view all these examples of dying trees as isolated cases, each caused by a specific insect or fungus or bacteria or virus or whatever. Or, more accurately, we can take the wider view and look on these pathogens as representing only the proximate causes of death, and see forest decline and tree death across eastern North America for the pandemic that it is. Our industrial society with its attendant air pollution is slowly killing our forests, as it has the forests of eastern Europe, weakening and stressing trees, and making them more susceptible to cold winters and attack by pathogens. Someone I was explaining the problem to recently made an analogy between acid rain and AIDS. You don't die of AIDS, you die of the diseases you contract because of your depressed immune system. The same holds true of atmospheric pollutants. It is often not the pollutants that kill the trees but the insects, viruses, fungi, or bacteria that attack once the overly-stressed trees' immune systems are weakened.

Once we begin to recognize the damage that is occurring, we then have to decide how long we are going to accept this transformation of our forests before we make changes in our own lifestyles and speak up to demand that the necessary changes be made in our society? In Germany the term waldsterben - or forest death - is now a household word. Is that what we want here? Is this trend of slow forest death what we want for our children's future, or is it time to rise up en masse to let our legislators know what we do want?

There is much that can be done to begin to solve the problem. To begin with, there needs to be much greater media coverage and public education on the issue. Beyond that, we can push for tighter air quality controls on industry and automobiles in Maine. We can put forth a citizen-initiated referendum to develop a statewide system of public transportation. And, since much of our air pollution originates from beyond our state boundaries, we can pressure Congress for tighter federal air quality controls. To quote ex-West German Chancellor Helmut Kohl, "If we do not succeed in saving our forests, the world in which we live will be changed beyond recognition."