Lichens
Lichens are
fungi that form symbiotic associations with photosynthetic green algal
or cyanobacterial cells, from which they obtain food. Approximately
20,000 species are recognized, most of which are ascolichens (the
fungus is a member of the Ascomycota); less than 1% of named lichen
fungi are members of the Basidiomycota. Most lichens have green algal
(phylum Chorophyta) photosythetic partners and the rest have
cyanobacteria alone or in combination with green algae.
Lichens are floristic components of most terrestrial ecosystems on
every continent, and the dominant vegetation on about 8% of the earth’s
surface (Nash, 1996). It is impossible to generalize about lichen
ecology since lichens inhabit terrestrial habitats ranging from arctic
tundra to tropical rainforests. As ‘plant-like’ fungi, however, all
lichens require light, so typical substrates include rocks, bark,
leaves and soil, nearly any surface that is exposed to adequate amounts
of light. This requirement for light means that lichen fungi are
frequently exposed to conditions uncommon for normal fungi. Lichens are
especially noteworthy for their tolerance of extreme temperatures and
irradiation levels. For example, lichens are one of only a few groups
of organisms that colonize substrates in hot and cold deserts, where
they may live both on the surface and even inside of rocks as
endolithic organisms (Friedmann, 1981, 1982; Green et al., 1999;
Wynn-Williams et al., 2000; Galun, 2001; de los Ríos et al.,
2005). A recent European
Space Agency experiment even demonstrated that certain lichens (Rhizocarpon
geographicum and Xanthoria elegans) exhibit no ill effects
after a 15-day exposure to open space (Sancho et al., 2007).
Despite their tolerance of extreme
natural conditions, however, many lichens are sensitive to air
pollution, which is why they are frequently used as biomonitors.
Lichen Biomonitoring
Lichens have long been used as
environmental monitors. The use of lichens as indicators of atmospheric
quality dates to the 1860's when independent observations in Europe established that lichen declines were
caused by air pollution. Studies done since have established that
lichens respond in predictable ways to changes in air quality, and over
1500 papers have been published (see reviews by Nash & Wirth 1988,
Nash 1989, Richardson 1992, Hyvärinen et al. 1993, Stolte et al.
1993, Gries 1996, Conti & Cecchetti 2001, Garty 2000, 2001, Nash
and Gries 2002, Nimis et al. 2002):
- Lichen floristic patterns change as sensitive
species are replaced by tolerant species. Especially
sensitive and tolerant species can therefore be used as bioindicators
of atmospheric quality.
- Sensitive species
exhibit structural, physiological and behavioral changes including
reduced photosynthesis, bleaching and death of the symbiotic algae, and
discoloration and reduced growth of the fungus.
- Tolerant species
are known to accumulate pollutant elements from the atmosphere and can
thereby sere as "passive monitors" of air pollution.
- When transplanted
to polluted sites, sensitive species exhibit physiological stress and
increased mortlity; tolerant species accumulate pollutant elements and
can also exhibit measurable changes in physiology and growth rate.
Lichen Biomonitoring in the
National Park Service (NPS)
Given their usefulness as
biomonitors, the NPS and the USDA Forest Service have each undertaken
numerous lichen studies on federal lands during the last thirty years
(Geiser & Reynolds 2002, Blett et al. 2003). For the NPS studies, a
useful summary of the lichen floristic data can be accessed at:
NPLichen, A
Database of Lichens in the U. S. National Parks
Lichen element data can be
accessed at:
NPElement,
A Database of Lichen Elemental Concentrations in the U. S. National
Parks
NPLichen lists over 29,000 records
(over 2,500 species total) of documented occurrences of lichens in 149
park units of the U. S. National Park System. NPElement lists element
data (48 elements total) for 75 lichen species surveyed in 43 park
units.
Lichen Biomonitoring in the National Capital Region
Lichens have been collected and
studied in the Washington, D.C. region since the late 1800's, and numerous
collections are available in the U.S. National Herbarium. Studies of
lichens as environmental monitors in the region began in 1965 when
Mason E. Hale, Jr., curator of lichens at the Smithsonian Institution,
initiated long-term growth-rate studies of rock-inhabiting lichens on Plummers Island, Maryland
in the Potomac River. Beginning in
1975, James Lawrey of George Mason University
joined Hale in these studies and additional study sites were
established at Great Falls, Maryland, Rock
Creek Park in Washington,
D.C., various other locations in
the Washington area, and in Shenandoah National
Park. In addition to lichen growth rates, Hale and
Lawrey began comparative studies of lichen floristic composition and
element accumulation.
These studies:
- established
permanent study sites at various locations in the Washingon, D.C. area.
- provided long-term
information about lichen growth rates and correlations with
enrvironmental variables (Hale 1970, Lawrey & Hale 1977, 1979).
- began to document
lichen floristic composition and element concentration at these
permanent sites (Lawrey 1991, 1992).
- provided informaton
about element uptake patterns and mechanisms (Hale and Lawrey 1985,
Schwartzman 1987, 1991)
- provided
comparative retrospective data for changes in lichen communities and
element content during the past 100 years (Lawrey & Hale 1981,
Lawrey 1993).
National S, N and Hg deposition
patterns
In eastern U.S.
nitrophilous, relatively pollution-tolerant lichen communities have
developed over time, probably the result of poor air quality in the
past and only slight improvement since. A recent study (McCune et al.
1997) analyzed lichen community patterns throughout the southeastern
U.S. as a part of the Forest Health Monitoring Program. The authors
found two major gradients in the data, one a macroclimatic gradient
from the coast to the Appalachian Mountains primarily related to
temperature, and the other correlated with air quality. Epiphytic
macrolichens were found to be sparse or absent in urban areas. In rural
areas, lichen communities are more luxuriant and contain a greater
number of pollution-senstive species. A similar, but more complicted,
trend was found for the northeastern U.S.
The National Atmospheric
Deposition Program provides information on pollutant deposition
patterns in the United States and the national deposition patterns for
sulfate and inorganic nitrogen (2005 data shown below) indicate that
the eastern U.S. is subject to generally high deposition of both types
of pollutants. This can result in simplification of lichen communities
through acidification and eutrophication. Lichens especially tolerant
of these changes will tend to dominate communities over time.
It is estimated that 60% of the mercury that circulates in the
atmosphere globally is of human origin, primarily from industrial,
power plant and incinerator emissions. The Tennessee
Valley Authority maintains a mercury information page
that summarizes sources of mercury in the U.S. A major concern about
mercury deposition is that it never breaks down, so it can be
recirculated in the environment long after its release. Since mercury
is a toxic heavy metal, there are concerns about atmospheric cycling
and deposition of mercury, and the potential consequences to human
health caused by mercury released into the environment. Human health
problems are usually caused by consumption of fish that have
bioaccumulated an especially toxic form of mercury, methylmercury, in
aquatic ecosystems. However, little is known about the health hazards
associated with atmospheric mercury.
Lichens have been used to monitor Hg emissions around factories, mines
and power plants in a large number of studies (Steinnes and Krog 1977,
Makholm and Bennett 1998, Bargagli et al. 1987, Bylinska et al. 1991,
Chilton and Orvos 2004, Ikingura and Akagi 2002, Krishna et al. 2003,
Lodenius and Laaksovirta 1979, Sensen and Richarson 2002). Lichens have
also been used to monitor natural sources of mercury deposition from
volcanoes, geysers, and other geothermal sources ( in the U.S. by
Bennett and Wetmore 1999, Davies and Notcutt 1996 and in Italy by
Bargagli and Barghigiani 1991, Dongarra and Varrica 1998, Grasso et al.
1999, Loppi 1996, Loppi and Bonini 2000).
The NADP Mercury Deposition
Network provides a database of Hg deposition patterns
nationally. Deposition patterns for total Hg are generally low for the
eastern U.S. However, Hg bioaccumulation can produce unusually
high levels in wildlife. A recent study of songbirds in N.Y. State
(reported in NY Times, July 25, 2006) showed high Hg levels, prompting New York to propose more stringent Hg emissions
levels for power plants.
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