Nice one icelid, I just found this.
Air Pollution, Lichens and Mosses
by Kevin J. Lyman
EDITOR'S NOTE: This article is reprinted without illustrations from LORE magazine, a benefit of museum membership. ©1996 Milwaukee Public Museum,
As early as the mid 1800's, botanists became aware that lichens and mosses were becoming uncommon in areas within and surrounding large towns and
cities. They began to recognize that air pollution emitted from these urban areas was affecting the colonization and growth of these organisms.
In 1866, William Nylander, a Finnish naturalist, was the first to link the disappearance of lichens and air pollution. He noticed that some lichen
species present within Luxembourg Gardens, Paris, were missing in other parts of the city. He attributed these differences to air quality. Over the
next thirty years, fumes from coal-burning industrial furnaces gradually led to the eradication of the lichen population within the park.
Along with lichens, mosses too have been disappearing from large cities since the late 1800's. Even though some species of mosses and lichens can be
found in the harshest environments (Antarctic, Arctic and deserts), most species are very sensitive to air pollution. There are, however, a few
species that can survive in areas where the pollution levels are relatively high and there are several other species that can tolerate moderate levels
of air pollution. By knowing which of these species are most sensitive to air pollution and documenting their presence or absence, it is easy to
determine how "clean" or "dirty" the air is.
Sulfur dioxide (SO2) does the most widespread damage to lower plants, even though it is only one of several air pollution components in the
atmosphere. Sulfur dioxide pollution is the result of industrial and urban emissions.
Why are mosses and lichens sensitive to air pollution? Since mosses and lichens lack roots, surface absorption of rainfall is the only means of
obtaining vital nutrients which are dissolved in rainwater. Lichens and many mosses lack protective surfaces that can selectively block out elements
including pollutants that are dissolved in rainwater.
Lichens act like sponges, taking in everything that is dissolved in the rainwater, and retaining it. Since there is no means of purging the SO2, the
sulfur content accumulates within the lichen and reaches a level where it breaks down the chlorophyll molecules which are responsible for
photosynthesis in the algae. Photosynthesis is the process green plants use to convert sunlight energy to chemical energy which in turn is used by the
plant. In the case of lichens, when the photosynthetic process stops in the algae, the algae die and this leads to the death of the fungus.
I'm off on a lichen hunt!