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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: [email protected] |
