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Lichens


by
James P. Bennett
National Biological Service
Lichens are a unique life form because they are actually two separate organisms, a fungus and an alga, living together in a symbiosis. Lichens seem to reproduce sexually, but what appears to be a fruiting structure is actually that of the fungal component. Consequently, lichens are classified by botanists as fungi, but are given their own lichen names.
Lichens are small plant-like organisms that grow just about everywhere: soils, tree trunks and branches, rocks and artificial stones, roofs, fences, walls, and even underwater. They are famous for surviving climatic extremes and are even the dominant vegetation in those habitats. Some lichens, however, are only found in very specialized habitats. The diversity of lichens in an area, therefore, is highly dependent on habitat diversity. Many special habitats across the United States are declining or disappearing because of human activities, and some lichen species are consequently in decline.
Lichens are very diverse in form: some grow flat and appressed to a substrate, others are more leaf-like and grow free of the substrate, and yet others have complex filamentous and blade-like forms.
Lichens are unique botanically because they lack any outside covering, or cuticle, and consequently are directly exposed to the atmosphere, which they depend upon for their nutrients and water, neither of which is derived from their hosts. Moistened lichen tissues act as blotters, soaking up chemicals or materials deposited on their surfaces. Unfortunately, this feature has also made them highly susceptible to air pollutants; lichens are perhaps the plant species most susceptible to sulfur dioxide, heavy metals, and acid rain.
Fig. 1. Cladina mitis and C. rangiferina (reindeer moss), Voyageurs National Park, MN. Courtesy J.P. Bennett
Lichens play important roles in ecosystems. They break down rocks and form soil by excreting weak acids, or in arid ecosystems like deserts, they help bind the soil surface by forming crusts. They are important food sources for invertebrates and vertebrates, including reindeer that eat reindeer "moss," which is actually a lichen (Fig. 1). In addition, some birds depend on certain lichens for nest-building materials. Finally, some lichens can fix nitrogen from the atmosphere and contribute a significant portion of this to certain forest ecosystems (e.g., the Pacific Northwest).
A rich lichen flora in a region indicates a lack of disturbance in the area for two reasons. First, lichens can only appear in an area if both the fungus and alga are propagated there and coincide. Isolation of an area so that propagules (see glossary) cannot reach the area will slow down recolonization significantly. Second, lichens grow slowly, usually only a few millimeters a year. Thus, colonization of an area by lichen species typically does not occur even over the span of one human generation.

Status
The best estimates of the number of U.S. lichen species are between 3,500 and 4,000, grouped in about 400 genera. The current checklist for the United States and Canada is probably in excess of 3,600 (Egan 1987).
Some species are cosmopolitan and are found from coast to coast. Most species, however, are more limited in their geographic distributions. The percentage of species that are rare nationally is high: about one-third of more than 400 lichens described by Hale (1979) are rare, and this ratio could probably be applied to the total number for the United States. Thirty-eight percent of the lichen flora of Hawaii is considered endemic. Five lichen species have been nominated for federal threatened and endangered listing (Pittam 1991), and several states (e.g., California, Minnesota, and Missouri) have listed some species as threatened or endangered.
No state has a complete lichen flora published. Incomplete floras or checklists are known for Alaska, California, Colorado, Connecticut, Florida, Hawaii, Louisiana, Michigan, Minnesota, North Carolina, New Mexico, New York, South Dakota, Tennessee, Texas, and Washington. Most of the rest of the country's lichen flora remains unexplored. Species for these partial checklists number in the several hundreds, with the exception of California with 999 taxa. Nationally, centers of diversity for lichens include the Pacific Northwest, California, the southern Appalachians, Florida, and Maine. On a more local scale, wetlands and floodplains tend to have higher numbers of lichen species than more arid areas. The presence of a bog or a rocky outcropping in an area will typically double the number of species present.
There are about 10 lichen herbarium collections with active lichen taxonomists in the United States, and about 5 in Canada. Many of these collections are poorly funded, not computerized, and stored in inadequate or outdated facilities. Fewer than two dozen practicing lichenologists work in the United States and Canada, and very few graduate students are being trained in lichenology. Most academic botany or biology departments do not have lichenologists.

Trends
About 100 years ago, lichens had disappeared from many cities in Europe and Great Britain and the term "lichen desert" was coined to describe the phenomenon; these lichen deserts were caused by air pollution. Here in the United States, lichen deserts are well known in our cities and nearby rural areas, but are unfortunately poorly documented. Most information is anecdotal, but some studies have shown lichen deserts in eastern Pennsylvania (Nash 1975), the Cuyahoga Valley in Ohio (Wetmore 1989), northern Indiana on Lake Michigan (Wetmore 1988), Cedar Rapids, Iowa (Saunders 1976), Los Angeles (Sigal and Nash 1983), Seattle, Washington (Johnson 1979), Copperhill, Tennessee (Mather 1978), and in Canada in Montreal (LeBlanc and De Sloover 1970) and Sudbury (LeBlanc et al. 1972) (Fig. 2). In some of these areas, researchers estimate that as much as 80%-90% of the original lichen flora is gone (Nash 1975; Wetmore 1989). Acid rain has diminished lichen diversity in remote rural areas such as north-central Pennsylvania (Showman and Long 1992), central and southwestern Connecticut (Metzler 1980), and southwestern Louisiana (Thompson et al. 1987). Sensitive species must be studied and monitored to determine the effects of air pollutants.

Fig. 2. Documented lichen deserts in the United States and Canada. Strong anecdotal evidence exists that lichen deserts also occur in most major cities.
Some lichens are unique to old-growth forests. Usnea longissima, which only grows in old-growth spruce forests, has vanished from many sites in western Europe (Esseen et al. 1992) and may be repeating this pattern in parts of the United States. Other old-growth forest lichens, including Alectoria sarmentosa, Lobaria scrobiculata, and Ramalina thrausta, are now quite rare in the eastern United States because of habitat destruction and loss.
In addition, scientific overcollecting may become a problem for lichens. One species, Gymnoderma lineare, was overcollected in Great Smoky Mountains National Park, Tennessee, in the late 1970's, and is now proposed for federal listing as endangered. Collecting is no longer allowed in certain areas (e.g., some national parks and nature preserves), and both the American Bryological and Lichenological Society and the British Lichenological Society do not always permit collecting at popular sites during their annual forays. Some hobby overcollecting of lichens for dye materials or architectural tree models is thought to be a problem in a few areas, but is not well documented.
Trends in lichenology in this country are not encouraging and are at odds with trends in the rest of world (Galloway 1992). With fewer universities offering training in the discipline, fewer surveys and lists of floras being done, less literature being published, and at the same time lichens disappearing from our ecosystems, it is clear that the science is heading the opposite direction of what is needed. Other countries, including England, Canada, the Netherlands, and Japan, are increasing funding for lichenology, training more students, publishing more literature, and conserving their lichen flora. Given the problems confronting lichen habitats, the size of the United States, and the potential flora it may have, lichen science needs more attention. A reasonable start would be a preliminary checklist for every state and an identification of priority areas for future surveys.
For further information:
James P. Bennett
National Biological Service
Wisconsin Cooperative
Research Unit
University of Wisconsin-Madison
Madison, WI 53705
References
Egan, R.S. 1987. A fifth checklist of the lichen-forming, lichenocolous and allied fungi of the continental United States and Canada. Bryologist 90:77-173.

Esseen, P.-A., B. Ehnstrom, L. Ericson, and K. Sjoberg. 1992. Boreal forests--the focal habitats of fennoscandia. Chapter 7 in L. Hansson, ed. Ecological principles of nature conservation. Elsevier, London.

Galloway, D.J. 1992. Biodiversity: a lichenological perspective. Biodiversity and Conservation 1:312-323.

Hale, M.E. 1979. How to know the lichens. 2nd ed. Wm. C. Brown Publishers, Dubuque, IA. 246 pp.

Johnson, D.W. 1979. Air pollution and the distribution of corticolous lichens in Seattle, Washington. Northwest Science 53(4):257-263.

LeBlanc, F., and J. De Sloover. 1970. Relation between industrialization and the distribution and growth of epiphytic lichens and mosses in Montreal. Canadian Journal of Botany 48:1485-1496.

LeBlanc, F., D.N. Rao, and G. Comeau. 1972. The epiphytic vegetation of Populus balsamifera and its significance as an air pollution indicator in Sudbury, Ontario. Canadian Journal of Botany 50:519-528.

Mather, T.C. 1978. Lichens as indicators of air pollution in the vicinity of Copperhill, Tennessee. Georgia Journal of Science 36:127-139

 Metzler, K.J. 1980. Lichens and air pollution: a study in Connecticut. Report of Investigations 9. State Geological and Natural History Survey of Connecticut. 30 pp.

Nash, T.H., III. 1975. Influence of effluents from a zinc factory on lichens. Ecological Monographs 45:183-198.

Pittam, S.K. 1991. The rare lichens project, a progress report. Evansia 8:45-47.

Saunders, J.R. 1976. The influence of SO2 on corticolous lichens in Cedar Rapids, Iowa. Submitted in partial fulfillment for College Honors (Biology) at Coe College, Cedar Rapids, IA, May 1976. 111 pp.

Showman, R.E., and R.P. Long. 1992. Lichen studies along a wet sulfate deposition gradient in Pennsylvania. Bryologist 95:166-170.

Sigal, L.L., and T.H. Nash III. 1983. Lichen communities on conifers in southern California mountains: an ecological survey relative to oxidant air pollution. Ecology 64:1343-1354.

Thompson, R.L., G.J. Ramelow, J.N. Beck, M.P. Langley, and J.C. Young. 1987. A study of airborne metals in Calcasieu Parish, Louisiana using the lichens, Parmelia praesorediosa and Ramalina stenospora. Water, Air and Soil Pollution 36:295-309.

Wetmore, C.M. 1988. Lichens and air quality in Indiana Dunes National Lakeshore. Mycotaxon 33:25-39.

Wetmore, C.M. 1989. Lichens and air quality in Cuyahoga Valley National Recreation Area, Ohio. Bryologist 92:273-281.


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