What's Wrong with the Picture on the Right?
Lichens retain what they absorb from air and water, and hence they are useful and cost-effective monitors of pollution. The U.S. Forest Service has initiated a program in Oregon and Washington to determine current lichen diversity and the levels of various air pollutants in lichens and mosses. Here (below left), a healthy red alder sports a dense coating of white lichens and scattered tufts of moss. It is in sharp contrast to the bare bark of another red alder (below right), downwind from a point source of air pollution.
Photos copyright S. Sharnoff