Editing Lichen (section)

=== Effects of air pollution ===
[[File:Lobaria pulmonaria 010108c.jpg|thumb|Some lichens, like the foliose ''[[Lobaria pulmonaria]]'', are sensitive to air pollution.]]
If lichens are exposed to air pollutants at all times, without any [[deciduous]] parts, they are unable to avoid the accumulation of pollutants. Also lacking [[stomata]] and a [[plant cuticle|cuticle]], lichens may absorb [[aerosol]]s and gases over the entire thallus surface from which they may readily [[diffusion|diffuse]] to the photobiont layer.<ref name=Nash2008/> Because lichens do not possess roots, their primary source of most [[chemical element|elements]] is the air, and therefore elemental levels in lichens often reflect the accumulated composition of ambient air. The processes by which atmospheric deposition occurs include [[fog]] and [[dew]], gaseous absorption, and dry deposition.<ref name=Knops1991/> Consequently, environmental studies with lichens emphasize their feasibility as effective [[bioindicator|biomonitors]] of atmospheric quality.<ref name=Nash2008/>

Not all lichens are equally sensitive to [[air pollution|air pollutants]], so different lichen species show different levels of sensitivity to specific atmospheric pollutants.<ref name="hogan">{{cite encyclopedia|last=Hogan |first=C. Michael |date=2010 |url=http://www.eoearth.org/article/Abiotic_factor?topic=49461 |title=Abiotic factor |encyclopedia=Encyclopedia of Earth. |publisher=National Council for Science and the Environment |location=Washington, D.C. |access-date=27 October 2013 |url-status=dead |archive-url=https://web.archive.org/web/20130608071757/http://www.eoearth.org/article/Abiotic_factor?topic=49461 |archive-date=8 June 2013 }}</ref> The sensitivity of a lichen to air pollution is directly related to the energy needs of the mycobiont, so that the stronger the dependency of the mycobiont on the photobiont, the more sensitive the lichen is to air pollution.<ref name=Beltman1980/> Upon exposure to air pollution, the photobiont may use metabolic energy for repair of its cellular structures that would otherwise be used for maintenance of its photosynthetic activity, therefore leaving less metabolic energy available for the mycobiont. The alteration of the balance between the photobiont and mycobiont can lead to the breakdown of the symbiotic association. Therefore, lichen decline may result not only from the accumulation of toxic substances, but also from altered nutrient supplies that favor one symbiont over the other.<ref name=Nash2008/>

This interaction between lichens and air pollution has been used as a means of monitoring air quality since 1859, with more systematic methods developed by [[William Nylander (botanist)|William Nylander]] in 1866.<ref name=WIL/>