Lichen and Air Quality
In 1866 the leading Finnish lichenologist William Nylander noticed that some lichens growing in the Jardin du Luxembourg in Paris didn't occur in any other part of the city. He realised that the lack of air pollution in the gardens was enabling the lichens to grow there, and that lichens are therefore an indicator of air quality.
Lichens had already been in use for years to test acidity. Litmus paper is made from the Ochrolechia tartarea and Rocella tinctoria varieties. Though Rocella is described as early as 1599, it was Ochrolechia that was first used to make the paper, in the 16th century in Holland, where manufacture continues to this day. The exact process is still kept secret, but involves ammonia, sodium carbonate, lime, and potash in a fermentation and extraction procedure.
There are four kinds of lichen. Foliose lichens are leaf-like; Fruticose lichens are bushy or shrubby; Squamulose lichens are scaly and may have stalks called podetia; and Crustose lichens are crusty. In general, the following table is a yardstick for measuring the local amount of sulphur dioxide (SO2), and hence the local air quality:
- Heavily polluted air - No lichen, only green algae
- Polluted air - Crustose lichen, e.g. Lecanora
- Moderate air - Squamulose and Podetial lichen
- Clean air - Foliose and Fruticose lichen
- Very clean air - Sensitive species such as Ramalina, Usnea, and Lobaria
But to get an accurate indication of pollution you'll need to identify specific species. Doing so can even identify the kind of pollution since, for example, Hypogymnia physodes is tolerant of SO2 and ozone but not fluoride, Lobaria pulmonaria is sensitive to SO2 but mildly tolerant of ozone, and Lecanora conizaeoides is a very toxitolerant bioindicator in general. The constant ebbing and flowing of pollution levels can provide some interesting situations:
The levels of air-borne sulphurous pollution have been dropping and this has enabled many species to return to areas where they have been absent for over 100 years. Indeed the appearance on concrete paving stone in Middlesex of the grey-green lichen Lecanora muralis caused some consternation. It was not recognised as a lichen by many people and correspondence to the press suggested that it had come from outer space!
Whilst Lecanora muralis is said to look like mutated chewing-gum, some species of lichen are in fact edible. The tree-hair lichen, Bryoria fremontii, was considered by some societies to be a food for times of famine, and by others a delicacy. In Japan, Umbilicaria esculenta is the most famous edible lichen. And Reindeer lichen, Cladonia rangiferina, gets its name from being edible to reindeer, but is in fact edible to humans too when properly cooked.
Cite: Palmer, S.B. (2005). "Lichen and Air Quality", in: What Planet is This?
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