Thoreau's exuberance at the lushness and endless fecundity of the Maine woods serves as the essential counterpart to his response to Katahdin's stormy, chaotic summit and the alarming sense of unwelcome he perceives there. Here is Nature once again in a form that delights and nourishes, even offers a man a chance to seek his place in eternity. Here is comfort for the eyes and food for the soul, with a terrific hike thrown in.

If he were to take that hike now, he would have a serious problem with the notion of the continuity of the forest. It is not so much a matter of patches of meadow cropping up where no meadow was before; rather, where once each tree sturdily filled its space with healthy foliage, now there are dead limbs and transparent crowns. Growth has slowed markedly. Far more trees have been lost to extreme weather, insects, and pathogens than were the case in Thoreau's day.

The term "acid rain" was first used in 1856 by Robert Angus Smith, a British chemist who discovered the increased acidity of precipitation falling near industrial Manchester.1 While this soon became a subject of concern in Europe, not until a century later did its effects become noticeable in the northeastern U.S.,2 the region downwind from the dirtiest of the nation's coal-fired power plants, as forests began to die back and lakes and streams lost entire species of fish. By 1970 air pollution in all its forms had become such an issue of concern that Congress was moved to pass the Clean Air Act; but the effects of acid rain continued to intensify. The Clean Air Act Amendments of 1990 took on acid rain more specifically, limiting emissions from smokestacks and vehicular exhaust pipes, and some areas began to show signs of improvement.3 But others did not.

A number of factors affect the impact of acid rain, including location, topography, climate, and the characteristics of the soil and bedrock in a particular area. Trees at high altitudes and along the coast are most affected because they are bathed in clouds or fog much of the time, and because they are already living at the limits of their range, subjected to high winds and severe winter weather or saltwater. The greater the stress on the trees, the more vulnerable they are to additional stressors, and the more likely they are to fall victim to disease, pests, or winter storms.

Acid rain attacks trees in several ways. It leaches important nutrients, like calcium, magnesium, and potassium, from the soil. It makes toxic metals that occur naturally in the soil, like aluminum and zinc, more absorbable. It does the same with heavy metals released into the air by the burning of fossil fuels and smelting, such as lead, mercury, and cadmium. Dioxins from incinerators also find their way into raindrops. So does ozone from vehicular exhaust, which damages chlorophyll.4

In much of Maine, where 90% of the land cover is woodland,5 growth continues to slow, then dieback sets in. Broadleaf and needle trees are both affected; among the trees suffering the most damage are sugar maple, green ash, and red spruce.6,7 Once the proportion of dead limbs reaches 15% or the crown becomes 55% transparent, the tree is officially in trouble.8 It won't become the great fallen log whose mossy decay offers such a fine metaphor for the ongoing cycle of life and death and life. Maine's Department of Environmental Protection offers hope, however: They maintain that an additional 80% reduction in emissions from power plants, coupled with greater controls on vehicle exhaust, could bring the forest back in 25 years.9 These are steep regulatory advances.

Helmut Kohl, the former Chancellor of Germany, a nation whose forests loom large in its history, culture, and mythology, declared in 1983, "If we can't save these forests, the world will become unrecognizable."10 Thoreau would make the case more richly, but the sentiment undoubtedly would be the same.

1 State of Maine, Department of Environmental Protection, Bureau of Air Quality. http://www.maine.gov/dep/air/acidrain/.

2 Likens, GE; Driscoll, CT; & Buso, DC. "Long-term effects of acid rain: response and recovery of a forest ecosystem." Science, New Series, 272, no. 5259 (1996): pp. 244-246.

3 State of Maine, Department of Environmental Protection, Bureau of Air Quality. http://www.maine.gov/dep/air/acidrain/.

4 Donahue, Paul. "Tree death and forest decline." Forest Ecology Network, March 1998. http://www.forestecologynetwork.org/tree_death.html.

5 Turkel, Tux. "Climate change in the Maine woods." MaineToday.com, March 4, 2007. http://business.mainetoday.com/news/070304climatechang.html.

6 Donahue, Paul. "Tree death and forest decline." Forest Ecology Network, March 1998. http://www.forestecologynetwork.org/tree_death.html.

7 Environmental Protection Agency. "Acid rain in New England." http://www.epa.gov/region1/eco/acidrain/enveffects.html.

8 Donahue, Paul. Op. cit.

9 State of Maine. Op. cit.

10 Kriener, Manfred. "A long walk in the woods: Germany and its unique relationship to its forests." The Atlantic Times, October 2008. http://www.atlantic-times.com/archive_detail. php?recordID=1501.