Forests in Decline

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Because the Germans have such a serious problem, they've been quick to search for the specific causes of forest decline. Though German scientists were ready initially to place the blame on acid rain and the aluminum it can liberate from a bound state in soil, a closer look pointed out a number of deficiencies in that theory. For one, forest damage has been found in acid and alkaline soils alike. What's more, an examination of soil chemistry showed that natural (humic) acids in the organic litter layer on top of forest soil could be a much more powerful influence than low-pH rainfall . . . particularly in soils that lack calcium carbonate for buffering. In fact, if a cubic yard of 3.5-pH rain were to fall on a square yard of forest soil covered with a two-inch layer of organic matter, the total acidity of the natural acids in the soil would be 500 times that of the rainfall. The effects of these natural acids, which might typically have a pH of 4.0 to 4.5, would easily outweigh the influence of acid rainfall, even though the rainfall might have a lower pH.

An interesting point to ponder concerning acid rain, however, is that sulfur dioxide gas (a precursor of sulfuric acid rainfall) can damage plant life. Before SO 2 ever becomes an acidic liquid, it can significantly stunt tree growth. These effects have been seen at concentrations of 25 to 50 micrograms per cubic meter-about half the U.S. standard for public health (80 micrograms per cubic meter). Thus the effects of sulfur dioxide pollution on trees could be most serious before the gas becomes acid precipitation.

Ozone is another potential contributor to forest decline. Excessive levels of O 3 produced from nitrogen oxide reacting in sunlight with hydrocarbons from car exhaust, woodstoves, etc.—have long been known to harm plants. At concentrations not even twice as great as natural background levels, ozone can degrade the waxy protective layer of leaf tissue, which leads to leakage of plant fluids. At present, this surface breakage is the strongest link known to widespread calcium and magnesium deficiencies. The most plausible explanation of these deficiencies—and of the attendant yellowing seen in so many forests today—is leaf damage from a combination of ozone and acidic water.

Ozone is particularly worrisome because it can damage foliage at concentrations of 100 to 200 micrograms per cubic meter if these conditions occur on six to eight successive days . . . conditions that, in the last 20 years, have been common over much of the earth's surface. (Oddly, swings into and out of the damaging concentration range are more destructive than consistently high levels. There is, however, almost always fluctuation, because the production of ozone requires sunlight.) Levels of O 3 in German forests are known to average 100 to 150 micrograms per cubic meter, and peaks in the 400 to 500 range aren't unusual. In the U.S., ozone is largely blamed for an 80% decline in forest productivity in the San Bernardino Mountains east of Los Angeles.

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