|EDITOR'S NOTE: This article is reprinted without illustrations from LORE magazine, a benefit of museum membership. ©1996 Milwaukee Public Museum, Inc.
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.
Many mosses assimilate much of their nutrients and water through their tiny leaves. Unlike the leaves of higher (flowering) plants which have a waxy covering called a cuticle, mosses lack this protection. This allows mosses to take in water quickly through their leaves during rains or high humidity, and dry out fast when the air dries. This rapid and direct absorption is detrimental when the moisture is laden with pollutants.
Since it is known that different species (especially 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.
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. 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.
Botany staff at the museum are actively working on a project involving lichens and air pollution. This project is a five year study currently being done with funding from the newly formed National Biological Survey. The study involves using a lichen species called Hypogymnia physodes which tolerates moderate levels of air pollution. This lichen was taken from a site in Door County, Wisconsin and transplanted to four sites along the Lake Michigan shoreline. The northern most transplant, which is also the control site, is in Door County. The southern most transplant is at Indiana Dunes National Lakeshore. The source site in Door County contains large numbers of Hypogymnia physodes, and the area has relatively low levels of air pollution. In the past, this lichen could be found all around Lake Michigan but it has perished in many places as the result of high levels of air pollution. For this study, the transplanted lichens, still attached to their original branches, were tied to artificial trees made of PVC pipe. Once a year, a few of the lichens are removed from the artificial trees and sent to a laboratory for chemical analysis. The chemical analyses show any additional accumulations or loss of SO2 when compared with the initial chemical analyses from the first year. After two years, it has been discovered that the SO2 levels have increased at the three sites south of Door County. The highest level being at Indiana Dunes, the lowest level at the Door County site. This project will reveal that if the lichens can survive, it is an indication that pollution levels in these areas are stabilizing or declining. In addition, valuable information will be obtained on the levels of several other pollutants, including sulfur, on the western and southern shore of Lake Michigan.
Lichens and mosses are sensitive to air pollution and have disappeared from many metropolitan and industrial areas over the last century. It is possible to take advantage of their sensitivity to pollutants and use them as biomonitors in transplant studies. Lichens and mosses 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.
WHAT ARE LICHENS?
Lichens (Latin, lichen, from the Greek word meaning "tree moss") are unusual organisms because they consist of fungal threads and microscopic green alga living together and functioning as a single organism. The main body of a lichen is called a thallus and does not resemble either the fungal or algal parts. Both components receive some benefit from this symbiotic association. Simply, the algae within the thallus manufacture sugars that the fungus can live off of and in return, the fungus provides protection for the alga. Lichens do not have roots, stems and leaves so they must receive their nutrients from rainfall.
Lichens grow readily and luxuriously on rocks, soil, trees or artificial structures in unpolluted habitats. Lichens can live in unfavorable terrestrial habitats throughout the world, including the Arctic, Antarctic and deserts. They are considered pioneer species in some habitats, because they are often the first organisms to invade newly exposed rock or soil. Following the colonization of a substrate, lichens may promote soil formation by adding organic material and dissolving minerals from the rock. In Wisconsin, there are approximately 565 different species of lichens.
WHAT ARE MOSSES?
Mosses belong to a group of green plants known as bryophytes (from the Greek words bryon, "moss" and phyton, "plant"). Bryophytes which include mosses, liverworts and hornworts, do not produce flowers, fruits or seeds. All reproduce by spores and can also form new plants from small fragments of stems and leaves that are broken off. Bryophytes have structures that resemble roots, stems and leaves but they lack true water and food conducting tissues.
Though relatively small, they can sometimes form a conspicuous component in many ecosystems. Commonly found growing on rocks, soil, rotting logs, trees and even shingles, bryophytes favor cool moist habitats like woodlands, stream banks and swamps. Mosses can withstand harsh environmental conditions as well and can be found in the Arctic, Antarctic and deserts. They also grow on sand dunes, where they play an important role stabilizing the shifting sands. Mosses along with lichens, are sometimes the first organism to colonize newly exposed soil. There are 532 species of bryophytes in Wisconsin of which 395 are mosses, 133 are liverworts, and 4 are hornworts.