Air Pollution, Lichens and Mosses
by Kevin J. Lyman
One can readily tell what the air quality is in their own backyard, park or woodlot just by looking at what is growing on the bark of older trees. (too bad our fair city loves to chop down older trees) Take some time and look at the bark of some older trees. Hopefully, a few scattered patches of gray or orange lichens can be seen growing on the bark. Farther away from a city, there is an obvious change in what is growing on tree trunks. A good place to see this is in the northern part of Wisconsin where the number of lichens and mosses growing on the trunks and branches of trees will be a lot higher. Yet there are still areas where air pollution levels are too high for lichens and mosses to grow. These areas are termed "lichen deserts" or "moss deserts." As the air quality in these lichen and moss deserts improve, both lichens and mosses will begin to reappear in a slow process of recovery.
More at http://www.mpm.edu/collections/pubs/botany/moss/