Something that should be noted: since lichens don't have the same protection mechanisms that the terrestrial plants have (waxy cuticle, stomata) they are very succeptible to airborne pollution. The pendulous lichen (Old Man's Beard) are very sensitive, and will not thrive if the air anything less than pure. On the other hand, there is a yellow lichen (xanthoria) that seems to thrive in urban areas where the air can be quite polluted.
If the tolerance levels of a particular lichen are known, then surveys can be taken which can give a rough estimate of the amount of air pollution in a given area. Also, it has been noted that lichen diversity (the number of lichen species in a particular area) will decrease when there is an increase in air pollution.
1. Richardson, D. H. S. Pollution monitoring with lichens.; Richmond Publishing Co. Ltd.:Slough, 1992