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Vol. 12 No. 4 - October 2006

Lichens and Air Pollution

By James P. Bennett

Lichens are small, non-vascular plants consisting of a fungus and an alga growing together in one tissue. The most commonly known lichens are those that are found on the bark of trees, or the reindeer lichens growing on the ground, but many other species grow on rocks, fences, roofs, tombstones, and other man-made objects.

Even though some lichens are extremely tough and grow in very inhospitable habitats, they are also notoriously sensitive to air pollutants, primarily sulfur dioxide and heavy metals. Lichen deserts, a phenomenon where lichens disappear from cities, were described over a hundred years ago and determined to be caused by sulfur pollution. Lichens are especially sensitive to air pollutants because they have no outer impermeable layer of tissue to exclude gases and particles that impair their metabolism. Consequently, accumulation of pollutants is greater than it is in the foliage of vascular plants, which have impermeable cuticles. Lichens accumulate unusually large amounts of deposits, including heavy metals, which eventually reach toxic concentrations.

Lichens are therefore excellent bioindicators and biomonitors. As bioindicators, the presence/absence of sensitive species is used to look for distribution patterns that reflect pollutant deposition. Voids in distributions may indicate whether lichens have died out due to heavy metals and/or sulfur oxide pollution. These observations are determined by conducting taxonomic inventories or surveys, which include sampling many species in many localities in the study area.

Lichens that do not die out, but are still present and are known to accumulate trace elements are used to indicate patterns of deposition. Common species that are found in most localities are used to facilitate collecting enough samples geographically. Many lichens are collected in bulk in a locality to avoid collecting an anomalous sample that might falsely suggest that a hot spot is present. The samples are typically cleaned of extraneous material, not washed, ground to a fine powder, and analyzed chemically for elements of interest. Typically, both nutritional and anthropogenic elements are included to evaluate the health of the lichens as well as for enabling a geographic study of deposition.

Hundreds of studies on air pollution have been done using lichens and have been published during the last one hundred years. Most of these studies have been carried out in Europe and North America where lichenology was given adequate attention for a long time. Air pollution, however, occurs world wide, and may be greater now in other countries as Europe and North America have passed air quality legislation and brought air pollution under control. Fortunately, in the past few decades, studies on lichens have also begun in other countries where air pollution is bad.

For example, using Recent Literature on Lichens on the Internet, I discovered five papers on air pollution and lichens from China, dating back to 1980. These studies have been done in Hong Kong, Nanjing, and Hangzhou. In India, six studies have been done in Lucknow, Faizabad and Kolkata starting about 1995. Africa is represented by two studies from Morocco and Tanzania just a few years ago. South America is well represented by 11 studies from Argentina, Bolivia, Brazil, Chile, and Venezuela dating from the mid-1980s. I even have one unpublished study from a national park in central Peru. It is good to see these studies appearing from the other major continents of the world. There are probably more studies than these that are not yet in the lichen literature database.

Why are the studies on lichens engaging the attention of researchers? There are two good reasons: (i) to increase our knowledge of lichens, and (ii) to do something that may help improve air quality. First, these studies add to our knowledge of lichen distribution. I have conducted or have been involved in dozens of such studies over the past 25 years. Most of these studies have been focused on lichen inventories. Most of these have been conducted in United States National Parks, and have been used to compile a database on lichens in the parks which is now on the World Wide Web (www.ies.wisc.edu/nplichen). Voids in lichen distribution have been detected in some parks as a result of this work.

Second, these studies have increased our knowledge of species sensitivities to air pollution. The presence/absence of species in areas with low or high levels of air pollutants and the element burden of species allows us to better understand the sensitivity of lichen to air pollution. The more we know about which species are where in the landscape, in cities, parks and industrial areas the better we will know which species may die out in the future. This is critical to understanding air pollution impacts and for regulating the pollution levels.

Finally, do these studies help improve air quality? From our experiences in the United States the answer is a qualified yes. Our country has many specially designated areas, including parks and wilderness areas, where good air quality is maintained by providing a high degree of protection to these areas. In these areas sensitive resources must be inventoried and monitored for impacts. Lichens have been included in these inventories, and many studies have been done in national forests and parks for this reason. This has helped the land managers understand the resources better, and to be better informed in making air quality decisions whenever necessary.

The studies on lichens with particular reference to impact of air pollutants on them are valuable to society and a well established scientific endeavor. This work should be supported wherever possible. Academia, government, industry, and the public are all involved and affected by this.

Gaylord Nelson Institute for Environmental Studies, U. S. Geological Survey and University of Wisconsin-Madison, U.S.A.

E-mail: jpbennet@wisc.edu

This article has been reproduced from the archives of EnviroNews - Newsletter of ISEB India.

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