When acid rain is absorbed into the ground, it breaks down natural minerals, such as calcium, potassium, and aluminum, and carries them into the substrata, thus robbing trees and plants of a vital source of nutrients.

Research now would seem to indicate that traces of toxic metals that are emitted into the atmosphere by fossil-fuel-burning plants and vehicle exhaust fumes combine with acid rain to destroy nutrients necessary to keep trees alive. Some authorities believe that because of continuing soil acidification, trace elements may be dissolved and absorbed by the roots and choke off a tree’s water supply.

Hans Mohr, a Freiburg University teacher, claims that the difficulty seems to be created by nitrogen, the very substance plants often have a shortage of. He refers to research indicating that nitrogen compounds in the atmosphere have increased by 50 percent over the past 20 years. This increase is due mainly to vehicle exhaust; static emission; gas-, oil-, and coal-fired central heating; and ammonia emission by agriculture and refuse-disposal facilities. Bernhard Ulrich of the University of Göttingen argues that the trees are not being strangled by what is in the air but are being poisoned by what is in the soil. Other authorities point an accusing finger at ozone levels, at the sinking water level, or at ineffective forestry policies.

“A new avenue of research,” observes Smithsonian magazine, “contends that the guilt falls on no single air pollutant, but rather on an as-yet unknown interaction among them, so that the whole impact is greater than the parts.” This is very possible. Almost 3,000 chemical compounds exist that can be called air pollutants. For several decades they have been building up in concentration, subjecting the forests to continuous stress.

As long as the ecological system was intact and strong enough to reject the effects of pollution, all went well. But now when causes for sickness, such as frost, drought, and insects, make their appearance, the trees find themselves too weak to resist.