Lichens are small, non-vascular plants consisting of a fungus and an alga growing together in one tissue. The most commonly known lichens are those that are found on the bark of trees, or the reindeer lichens growing on the ground, but many other species grow on rocks, fences, roofs, tombstones, and other man-made objects.

Even though some lichens are extremely tough and grow in very inhospitable habitats, they are also notoriously sensitive to air pollutants, primarily sulfur dioxide and heavy metals.

Lichens are especially sensitive to air pollutants because they have no outer impermeable layer of tissue to exclude gases and particles that impair their metabolism.

Consequently, accumulation of pollutants is greater than it is in the foliage of vascular plants, which have impermeable cuticles. Lichens accumulate unusually large amounts of deposits, including heavy metals, which eventually reach toxic concentrations.

Lichens are therefore excellent bioindicators and biomonitors. As bioindicators, the presence/absence of sensitive species is used to look for distribution patterns that reflect pollutant deposition. 

Hundreds of studies on air pollution have been done using lichens and have been published during the last one hundred years. Most of these studies have been carried out in Europe and North America where lichenology was given adequate attention for a long time.

Studies carried out in the 1970s in Europe were able to create a graded series of 10 zones with indicator species for each zone, varying from highly polluted to basically unpolluted or pristine. The first category was no lichens, the second contained algae only, the third zone has Hypogymnia physodes and Parmelia caperata.

The last zone was characterised by rare species such as Usnea articulata and Teleoschistes flavicans.
These studies have helped improve air quality in the US.

Compiled by Darren D’mello