Conservation
Lichens measure air quality
Lichens draw water and salts straight from the air, so they respond to its composition. They have served as living pollution sensors for over a century.
A lichen is a durable association of a fungus with a photosynthetic partner — an alga or a cyanobacterium — together with associated microorganisms. It has no roots taking water from the soil, and no sealed layer separating the thallus from the atmosphere. Water and dissolved substances arrive directly from precipitation and from the air, which is why the composition of lichen communities responds to pollution.
Historically, sulphur dioxide was particularly important. Sensitive species disappeared from industrial centres, and zones of ever poorer lichen flora formed around emission sources. After sulphur emissions were reduced, some species returned. Today the deposition of nitrogen compounds may play a larger role, favouring different sets of species and altering the reaction of the substrate.
A "lichen scale" is not a home instrument giving an exact concentration. The result also depends on the species and age of the tree, the pH of the bark, humidity, light and the history of the place. Professional biomonitoring uses an established protocol, comparable plots and correct identification. A single lichen on one trunk does not prove the air is clean.
What lichens can do is integrate conditions over time, whereas an instrument measures a particular moment and a particular compound. Analysing community composition and the substances accumulated in thalli complements instrumental measurement. For a walker, the varied forms — crustose, foliose and fruticose — are an invitation to observe, but assessing air quality requires a method, not merely the presence of a green patch.
The indicator is a community, not one species
Geiser and colleagues analysed 127,001 voucher specimens, 572 species and 8,855 US forest plots. Models related nitrogen and sulfur deposition to total richness, sensitive species, cyanolichens and community composition. Critical loads for a 20% decline in sensitive-species richness were 3.1 kg N ha⁻¹ yr⁻¹ and 2.5 kg S ha⁻¹ yr⁻¹; the current NPS method uses 3.1 and 2.3 as conservative condition thresholds. These are annual deposition loads, not momentary air concentrations. The calibration concerns epiphytic macrolichens in US forests, so its numbers cannot simply be transferred to a Polish city or a rock-dwelling lichen. Lichens complement instrumental stations rather than replacing them.
Written by MykoRadar from the source indicated. Informational only — it does not replace advice from an expert.