A lichen is a plant. It has no leaves, stems or roots, but like other plants it makes its own food using energy from sunlight. Lichens resemble mosses: both are small and grow on trees or rocks as well as on the ground. Mosses are made up of slender, green stems with tiny, transparent, green leaves. Lichens, however, come in many forms: paint-like rusts; scalloped, wrinkled sheets; lace-like pads; bushy tufts; unkempt strands of black, ray or green "hair"; but they are never made up of sterns and leaves. A lichen is a fungus, but it contains one or more kinds of algae which make its food. The body of a lichen is formed by the fungus. It consists of materials similar to those in a mushroom. Inside the lichen is a layer of green or blue-green algae. The algae make food, both for themselves and for the fungus. The partnership between the algae and fungi in lichens is a classical example of a relationship known as symbiosis.
Lichens which grow on trees and shrubs do not harm them. Although the lichens are attached to the bark or penetrate a short distance, they do not enter the inner bark where food is transported, and hence do not rob the tree of nourishment. Neither do lichens cause disease. In winter in the Willamette Valley, the oak and ash trees seem to be choked with lichens. Although we might expect these lichens to shade the leaves and thus harm the trees, this does not occur. In the spring when new leaves emerge, they grow only at the ends of twigs, where there are no lichens. Thus, the leaves grow on the outside of the tree and shade the lichens on the older twigs and branches, rather than the lichens shading the leaves. In addition, lichens grow in the winter, after the leaves have fallen from the trees and no longer block light from reaching the lichens. Thus, the leaves and lichens take turns: leaves use sunlight in summer, lichens use it in winter.
Lichens, like other plants, need water and minerals in order to grow. Lichens growing on trees get both water and minerals from. The air. When a lichen is wet from rain or dew it grows actively, but when it dries out in summer, it stops growing. However, the lichen does not die, but instead lies dormant until the next rain starts it growing again.
Minerals in the air such as tiny, dust-like particles of soil are carried by the wind to the surface of lichens. There, dissolved in rainwater, they are taken up by the lichen and used for growth. Small amounts of airborne minerals, the amounts found in clean air, are beneficial to the lichens. However, it is possible to get too much of a "good" thing. Large amounts of minerals, such as the amounts found in polluted air, will poison the lichens and kill them.
Many kinds of pollutants are found in the air. Some, like sulfur dioxide, are gases. Others, like carbon, occur as very small particles. Some pollutants are found at low levels in clean air and only become annoying or poisonous when they occur at abnormally high levels. Others are seldom, if ever, found in clean air.
Since air pollution occurs in so many forms, it is difficult to design equipment which can measure all the types of pollution and measure it everywhere, day and night. As a consequence, most measurement is done near known sources of pollution and is limited to measurement of only one kind of pollutant.
Whether we live in a city or way out in the country, each of us would like to have the assurance that the air in our own neighborhoods is clean. We need a measuring device that is cheap, that can be used anywhere, and that responds to many kinds of airborne pollutants. Lichens, especially those which grow on trees, provide a partial but very useful substitute. They are cheap, they can be found in most areas of the Willamette Valley, and they are sensitive to many different kinds of pollutants. This guide is intended to show you how to use lichens to measure air pollution.
More than a century ago William Nylander, a European scientist, noticed that lichens found on trees in the countryside around Paris were not found on the same kinds of trees inside the city. He thought that lichens had grown there once, but had later been killed by pollutants.
In the past ten years scientists in Sweden, Great Britain, Canada and the United States have studied the effects of airborne pollution on lichens and have developed ways to use lichens to detect pollution. (A selected bibliography of technical reports of this work is included in this guide.) These scientists have found that some kinds of lichens are more easily killed by air pollution than others. Thus, in places where the air is very dirty, no lichens survive; in areas with slightly cleaner air one or two very resistant lichens can grow; in cleaner areas five or six species are found in the cleanest areas a dozen or more species of lichen thrive. By learning which lichens are most sensitive to air pollution and which ones are most resistant, you can judge the quality of the air by examining the lichens.
The scale used in this GUIDE is based on data from 170 sites in the Willamette Valley. The two most resistant (Class 1) lichens, Xanthoria polycarpa and Parmelia sulcata, were found almost everywhere except for downtown Portland and a very few isolated industrial sites. In areas with slightly cleaner air, these two species were accompanied by three slightly more sensitive (Class 2) lichens: Ramalina farinacea, Parmelia subaurifera, and Usnea subfioridana. In still cleaner areas these species as well as the still more sensitive Class 3 lichens (Lepraria membranacea and Evernia prunastri) were present. Finally, in the cleanest areas Class 4 lichens (Lobaria pulmonaria and Physcia aipolia) were found in addition to the other species. In evaluating the air quality of a site, one should note the most sensitive lichens present (those of the highest class), because they indicate the air quality at that site. If no lichens are present, the air is very dirty. If only Class lichens are present, the air quality is only slightly better. However, if all classes of lichens are present, including the very sensitive Class 4 lichens, then the air is very clean.
It should be noted that the lichen method will not tell you how much of a particular pollutant is present. It is only an indicator, although a very sensitive indicator, which can reassure you when the air is clean and warn you when it is not. Other methods must be used to determine which pollutants are present and to measure the exact amount.
Environmental factors other than air quality determine whether a lichen can grow in a given spot. By using only the lichens on branches of oak and ash trees we can limit, or even eliminate) the influence of factors other than air quality.
The chemical composition of the substrate, the surface on which a lichen grows, determines which lichens, if any, can grow there. For example, some species of lichens grow on granitic rocks, while others grow only on limestone. The chemical composition of the bark of branches of oak and ash trees varies little from place to place in the Willamette Valley. Therefore, differences we observe in the lichen growing on branches are not caused by these negligible differences bark composition. There are major differences in amounts of light and moisture different sides of a tree trunk. Moisture varies because rainwater flows down channels in the bark of the trunk, leaving intervening are relatively dry. On the other hand, small branches well up in the tree receive similar amounts of light and moisture whether they are on the north or the south side of the tree. By examining the lichens branches we can limit differences caused by variation in light and moisture.
The kinds of lichens found on an exposed surface are determined in part, by how long the surface has been exposed. New twigs have no lichens growing on them. Those lichens which do grow after a year or two are replaced after a while by different species and are in turn replaced by others. By examining the lichens on branches of the same age, we can eliminate differences due to length of time of exposure.
Every branch is composed of segments of different ages. The tips of the twigs grew this year; the next segment grew last year; the next grew the year before that, and so on. The boundary between one year's growth and the next is marked by a ring of scars which encircle the twig (see Fig. 3). These are the scars of bud scales which formed a terminal bud protecting the growing point of the twig during the winter, and which fell off, leaving the scars, when the twig resumed growth the following spring. We can determine the age of any segment by counting the number of rings of scars between it and the tip of the twig, since each ring of scars represents a winter that has passed since the segment was formed.