What do lichens indicate

Lichens also absorb sulphur dioxide dissolved in water. Lichens are widely used as environmental indicators or bio-indicators. If air is very badly polluted with sulphur dioxide there may be no lichens present, just green algae may be found.

If the air is clean, shrubby, hairy and leafy lichens become abundant. A few lichen species can tolerate quite high levels of pollution and are commonly found on pavements, walls and tree bark in urban areas. The most sensitive lichens are shrubby and leafy while the most tolerant lichens are all crusty in appearance. Since industrialisation many of the shrubby and leafy lichens such as Ramalina, Usnea and Lobaria species have very limited ranges, often being confined to the parts of Britain with the purest air such as northern and western Scotland and Devon and Cornwall.

Acid rain became a recognised international problem during the s resulting from the dispersion of air pollutants via tall chimney stacks. Air pollution and acid deposition has led to problems for lichens on bark, particularly because the tree bark has often become more acidic. In some areas, although gaseous sulphur dioxide levels have fallen, the bark of older trees is too acidic for recolonisation, and new growth develops on twigs and younger trees.

Some species of lichens have become more widely distributed than they were a century ago as they are more tolerant of acid conditions, such as some species of Bryoria, Parmeliopsis, Pseudevernia and Rinodina. The most common source of air pollution is the combustion of fossil fuels. This usually happens in vehicle engines and power stations.

Sulphur dioxide is released if the fuel contains sulphur compounds. This gas contributes to acid rain. Lichens can be used as air pollution indicators, especially of the concentration of sulphur dioxide in the atmosphere. The fungus provides shelter for the algae and the algae provides food for the fungi. Lichens do not have roots; instead they receive all their nutrients from the atmosphere.

Lichens as Bio-Indicators Lichens are sensitive to atmospheric pollution such as nitrogen N because they receive all their nutrients and water from wet and dry atmospheric deposition fall out. Nitrogen deposition can increase the load of nutrients. Certain species of lichen are more tolerant of N than others. Scientists monitor lichen communities.

Both the lichen constituents are affected by gaseous and particulate pollutants. Matter accumulates in the tissues bringin bout degenergation and a number of morphological changes. Mitra and Santra also discovered more pollution-tolerant species, like Bacidia convexula and Parmelia caperato, are found in areas with higher sulphur dioxide levels, while more sensitive species like TrypetheIium trot m and Gyrostomum scyphiliferum are found where pollution levels are lower.

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