Lichens as bioindicators of air pollution

Sensitivity and tolerance

Certain lichen species are especially sensitive to air pollution, while others are especially tolerant. For example, many species are sensitive to even moderate levels of SO2 pollution and rapidly disappear from polluted habitats. However, the "pollution lichen" Lecanora conizaeoides is very tolerant of SO2 and other pollutants, and readily invades habitats vacated by sensitive species. Especially sensitive or tolerant species are referred to as "indicator species" because their presence or absences can be a relatively accurate predictor of the air quality. Fumigation experiments with whole lichens and isolated symbionts have established sensitivities for many lichens to SO2, NOx, HF, ozone and metals. These studies have shown that even low concentrations of pollutants can be damaging to sensitive species.

Lichen community composition can also indicate changes in air quality. In the United States, changes in lichen communities have been correlated with changes in air quality in southern California (Nash and Sigal 1998), Seattle (Johnson 1979), Indianapolis (McCune 1988), and the Ohio River Valley (Showman 1990, 1997).

A very good survey of this literature can be found at:

Air Quality and Lichens - A literature review emphasizing the Pacific Northwest, U.S.A.

Establishing the pollution sensitivity of lichens requires both field and laboratory study, and the following approaches are commonly taken:

  • floristics and distribution mapping
  • gradient analysis along known pollution or natural environmental gradients
  • fumigation studies
  • element accumulation
  • transplants
  • photography

Based on results of these studies, indices of lichen sensitivity have been developed to provide a measure of the air quality related values of information from lichen surverys. The most familiar published lichen sensitivity scales are those of Hawksworth and Rose (1970), de Wit (1976), Wirth (1991), and Insarova et al. (1992), all for European lichens. These scales are based on correlations between lichen distribution data and air quality monitoring data. In the Netherlands, where there is a country-wide, high-density air quality monitoring network, very accurate modeling of lichen responses to air quality has been done (van Dobben and ter Braak 1999).

In the United States, sensitivity ratings are based on field data, literature surveys and fumigation experiments. Commonly cited sensitivity scales for U.S. lichens are those of LeBanc and De Sloover (1970), Sigal and Nash (1983), Wetmore (1983), McCune and Geiser (1997) and Peterson et al. (1992). The lichen communities of the Pacific Northwest have been studied most thoroughly, and lichen sensitivities have been established for numerous species. A summary of this information can be found at:

The US Forest Service National Lichens and Air Quality Database and Clearinghouse.

An illustrated listing of species and sensitivities is also available at:

Pacific Northwest Lichen Sensitivity Ratings by Species.

Categories of lichen biomonitors

Lichens can be assigned to various functional groups based on their ecosystem roles and responses to various pollutants. McCune et al. (2006) provide an overview and discussion of these groups for the Sierra Nevada National Parks, and we include those most useful for pollution monitoring in the NCR here. Certain lichens are also known to be especially pollution-sensitive or pollution-tolerant. Studies in the Pacific Northwest have helped to identify lichens that are especially sensitive or tolerant of pollution, and a provisional air quality rating system has been developed for numerous species. The categories may overlap since, for example, nitrophiles or acidiphiles may also respond positively or negatively to pollution:

  • Pollution-sensitive-- species that generally respond negatively to a wide range of pollutants. In the NCR study, these inlcude N-fixing cyanolichens (discussed below), Ramalina spp., Tuckermannopsis spp., and Usnea spp.
  • Pollution-tolerant-- species that generally respond positively to a wide range of pollutants. In the NCR study areas, these include Physcia millegrana (one of the most pollution tolerant lichens in the U.S., McCune 2000), Candelaria concolor, Parmelia sulcata, and Punctelia rudecta.
  • Nitrophiles-- these species thrive in nutrient-enriched areas receiving N inputs from fertilizer application in agricultural areas or N emissions from power plants, automobile exhaust or industry (van Herk 1999). Among the most common in the NCR are Candelaria concolor, Flavoparmelia caperata, Flavopunctelia flaventior, Parmelia sulcata, Phaeophyscia orbicularis, Physcia aipolia, Physcia millegrana, Punctelia rudecta and P. subrudecta. These are the most common lichens in the NCR plots, and can be found even in park units nearest the center of Washington, D.C. (NAMA, NACE).
  • Acidophiles-- these species thrive on acidic substrates, some natural (such as Parmeliopsis spp. on conifer bark), others (such as Lecanora conizaeoides) affected by acid deposition (van Dijk 1988, van Herk 1990). Few of these lichens are found in the NCR, which appears to be affected most by eutrophication.
  • Nitrogen-fixers-- these species contain cyanobacteria that fix atmospheric N. They are most commonly collected from shady, moss-covered rocks, soil and tree bases. Most are sensitive to all forms of pollution, especially SOx and NOx deposition, but also ozone. Herbarium records indicate that cyanolichens were common the Washington, D.C. area in the past. Some species (Coccocarpia palmicola, Collema furfuraceum, Leptogium cyanescens, Peltigera spp.) are still found occasionally in plots throughout the NCR study area, usually off-frame (below 0.5 m on trees or on soil). However, they are absent from park areas nearest the center of Washington, D.C. (NAMA, NACE).
  • Lichen parasites-- Some fungi, known as lichenicolous fungi, are known to grow exclusively on lichens. These should be included in lichen surveys since they may respond positively or negatively to changes in atmospheric quality.


NCR Lichens Home



Lecanora conizaeoides, a pollution-tolerant acidophilous crustose lichen.
Image © 2007 by Norbert Stapper from http://www.lichenology.info.

F caperata

Flavoparmelia caperata, a nitrophilous foliose lichen used for elemental analysis.
Image © 2006 by Paul Diederich from http://www.lichenology.info.

Usnea ceratina

Usnea ceratina, an acidophilous, pollution-sensitive fruticose lichen.
Image © 2006 by Paul Diederich from http://www.lichenology.info.


Peltigera praetextata, a pollution-sensitive cyanolichen.
Image © 2004 by Paul Diederich from http://www.lichenology.info.


Indicator species in the NCR

Certain species of lichens collected in the NCR study have been tentatively assigned to indicator categories based on published reports. These categories include the indicator and functional groups listed above. It should be noted that a specific lichen may be assigned to more than one grouping.  (Key: S = sensitive; T = tolerant; N = nitrophilous; A = acidophilous; C = cyanolichen; para = lichenicolous fungi):

Allocetraria oakesiana  S Heterodermia speciosa Parmotrema stuppeum Punctelia rudecta  NT
Anaptychia palmulata Hypotrachyna livida  S Parmotrema tinctorum Punctelia subrudecta  NT
Candelaria concolor  N Leptogium cyanescens  CS Phaeophyscia adiostola Pyxine caesiopruinosa
Canoparmelia caroliniana Myelochroa aurulenta Phaeophyscia orbicularis  N Pyxine sorediata
Cetrelia olivetorum  S Myelochroa galbina Phaeophyscia pusilloides Rimelia reticulata
Coccocarpia palmicola  CS Parmelia squarrosa  S Phaeophyscia rubropulchra N Tuckermannopsis ciliaris  AS
Collema furfuraceum  CS Parmelia sulcata  NT Phaeophyscia squarrosa Usnea ceratina  AS
Dirinaria aegialita Parmelinopsis horrescens Physcia aipolia  N Usnea strigosa  A
Flavoparmelia caperata  N Parmelinopsis minarum Physcia americana Usnea sp.  AS
Flavopunctelia flaventior  N Parmotrema dilatatum Physcia millegrana  NT Marchandiomyces corallinus para
Flavopunctelia soredica  Parmotrema hypotropum  T Physcia stellaris
Athelia arachnoidea  para
Heterodermia obscurata Parmotrema michauxianum Physconia detersa Nectriopsis parmeliae  para