Lichens are injured by sulfur dioxide (SO²). Rose (1975) has calculated that more than one-third of England and Wales has lost nearly all its epiphitic lichens, the most delicate shrubby lichens, largely due to the sulphur-dioxide emissions of coal-burning power plants. In Northern Siberia, an area of the Soviet Union which is very polluted, the number of lichen species has fallen from 50 to about 3, and the lichen production in general stands at about 1 or 2% or normal levels, threatening the reindeer diet; in Alaska there are similar concerns about lichen reduction and the caribou diet. (Tyson, 1990).

Losses in other parts of the world reflect the increasingly poor quality of the earth's air and the need for early warning bioindicators such as lichens.

This pollutant has natural sources, such as volcanic eruptions and sea spray. By far the largest source for it, however, is the combustion of fossil fuels, automobile emissions, and some industrial processes. The pollutant is carried in the atmosphere until rained out or deposited as dry particles or as gas. Sulfur dioxide combines with moisture in the atmosphere to form sulfurous acid (H²SO³) or sulfuric acid (H²SO⁴). When this happens with rainwater, the result is acid rain. All these forms of sulfur are harmful to lichens and plants.

Lichens have also shown sensitivity to some other pollutants, such as heavy metals and ozone, but for the most part lichen damage can be attributed to SO².

The effect of pollution upon lichen depends on the pH of the substrate, the surface on which the lichen grows. In general, an alkaline substrate such as basic bark or limestone counteracts the acidity of SO² pollution. As acid rain falls on a substrate, one kind of lichen growth form will often be replaced by another more tolerant form. In areas of high pollution lichens may be found only on sites such as wounds on trees and on sandstone walls, which have high (basic) pH.

Scientists have found that, with considerable SO² pollution in an area:

• The first loss of the same pH-sensitive lichens occurs on birches and conifers (acid bark and low buffering capacity);
• The next loss on oaks and sycamore (intermediate acidity and buffering capacity);
• The last on trees like elm (alkaline bark and high buffering capacity).

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