Our Livini^ Rt'soiirces — Plants 



195 



Lichens are unique bolunicaliy because they 

 lack any outside covering, or cuticle, and conse- 

 quently 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. L'nRirtunately. this feature has 

 also made them highly susceptible to air pollu- 

 tants: lichens are perhaps the plant species most 

 susceptible to sulfur dioxide, heavy metals, and 

 acid rain. 



Lichens play important roles in ecosystems. 

 They break down rocks and form soil by excret- 

 ing weak acids, or in arid ecosystems like 

 deserts, they help bind the soil surface by form- 

 ing 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 mil- 

 limeters 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 cuiTent 

 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, how- 

 ever, are more limited in their geographic distri- 

 butions. 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. Thiity-eight 

 percent of the lichen flora of Hawaii is consid- 

 ered endemic. Five lichen species have been 

 nominated for federal threatened and endan- 

 gered listing (Pittam 1991). and several states 

 (e.g.. California, Minnesota, and Missouri) have 

 listed some species as threatened or endan- 

 gered. 



No state has a complete lichen flora pub- 

 lished. Incomplete floras or checklists are 





known for Alaska. Calilornia. 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 tloodplains 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 dou- 

 ble the number of species present. 



There are about 10 lichen herbarium collec- 

 tions with active lichen taxonomists in the 

 United States, and about 5 in Canada. Many of 

 these collections are poorly funded, not com- 

 puterized, 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 yeai's ago, lichens had disap- 

 peared 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 deseils are well known in 

 our cities and nearby rural areas, but are unfor- 

 tunately 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). 



Fig. 1. Ckidina mitis and C. 

 rangiferuHi (reindeer moss), 

 Vovaaenrs National Park. MN. 



