Lichens are both beautiful and good. They indicate healthy air.
Last month when I was helping some friend assess their trees they were worried about the lichens “eating away” at their fruit trees’ wood. Assuming they were parasitic, he had used a wire brush to remove all the lichens from one tree. It jogged my memory.
Such trees are covered with a wonderful patina of various colors and shapes of lichens. The lichens take so many fascinating forms: circles of warm orange, furry gray-brown wavy circular fans, wavy greenish-brown circles, and the many other curious forms.
My friends were worried that a horrible fungus has invaded the trees’ living tissues. Fortunately, lichens are not parasitic. They live on the outer bark without effecting the living cells. (A lichen is an odd combination of both fungal filaments algae cells, usually a green algae.)
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.
Sulfur dioxide (SO2), the result of industrial and urban emissions, does the most widespread damage to lower plants, even though it is only one of several air pollution components in the atmosphere.
Why are lichens sensitive to air pollution? Since lichens lack roots, surface absorption of rainfall is the only means of obtaining vital nutrients which 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. When the photosynthetic process stops in the algae, the algae die and this leads to the death of the fungus.
Since it is known that different species of lichens vary in sensitivity to air pollution, scientists can use these organisms as monitors of air pollution and as indicators of air quality. This is very useful because modern air quality instruments cannot measure the effects air pollution has on living cells and they are limited to measuring present conditions.
Most importantly, the lack of lichens on fruit trees would be a sure indicator that their orchard is not free from the harsh chemical sprays that harm and kill lichens.
I could readily tell that the air quality was fine in their backyard just by looking at what is growing on the bark of older trees. Take some time and look at the bark of some of your older trees. Hopefully, a few scattered patches of gray or orange lichens can be seen growing on the bark. Near a city, there is an obvious change in what is growing on tree trunks. Here there are areas where lichens don’t exist, such areas are termed "lichen deserts". As the air quality in these lichens deserts improve, lichens will begin to reappear in a slow process of recovery.
Lichens are sensitive to air pollution and have disappeared from many metropolitan and industrial areas over the last century. Lichens’ sensitivity to pollutants are actually used as biomonitors—like a green version of a canary in the coal mine.. Lichens are valuable research tools and through the information they provide, we can have a better understanding of the impact air pollution has on the environment.
There many books on the subject. Here are two:
T.H. Nash (Ed.). Lichen biology. Cambridge University, Press, 1996. Pp. 303
ISBN 0 521 45368 2
"This book is in the tradition of the work of ME Hale 'The biology of lichens', the third edition of which appeared in 1983 (first published in 1967). It provides a comprehensive and up-to-date account of the fascinating world of lichens. The last chapter explains the role of lichens as indicators of air pollution and the pollutant effects on lichen biology is discussed."
Another book with less jargon is: By D. H. S. Richardson, Paperback; Naturalist's Handbook Series: 19. Pollution monitoring with lichens. Published by Richmond Publishing Company, 1992, ISBN 978- 0855462895
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