Dr Holger Thüs
Lichen Curator, Natural History Museum

Presented by La Sainte Union Catholic School, The Natural History Museum, The British Lichen Society, Camden Local Authority (Air Quality) and Imperial College

Pat Wolseley
Scientific Associate, The Natural History Museum

Further reading

  • Dobson F. 2006 Guide to Common Urban Lichens: On Trees and Wood Part 1. Field Studies Council.
  • Purvis W.  2007 Lichens. The Natural History Museum.

Lichens can be used as indicators of changes in air quality since tolerant species replace species which are sensitive to a given pollutant. This effect is observed widely across urban and rural Britain. It is especially apparent in regions where oxidised forms of nitrogen, particularly nitrogen dioxide, have become the dominant pollutants in the atmosphere, as often occurs in cities.

Lichen -4-320Students measuring the tree girth

This exhibit is presented by La Sainte Union Catholic School, which was awarded a Partnership Grant by the Royal Society in 2011 to investigate the relationship between air quality and lichen distribution.

How it works

The scientists behind this exhibit wanted to investigate the relationship between small scale differences in air quality, particularly the levels of NO2, nitrogen dioxide, and the lichen species found on nearby trees. 

Lichens can be placed into three categories, which can be used to quantify pollution levels. This is known as the OPAL pollution index. Nitrogen-sensitive lichen (e.g. Evernia) is found in clean, non-polluted conditions; intermediate lichen (e.g. Flavoparmelia) can be found in clean and polluted conditions; and nitrogen-loving lichen (e.g. Leafy Xanthoria) is found in conditions where levels of nitrogen dioxide are particularly high.

When suitable sites had been selected, the frequency of lichens on trees was recorded and levels of NO2 were measured with diffusion tubes. The study found that lichen diversity varies with tree species and their location, and that the OPAL pollution index correlated with measured NO2 levels. It showed that lichens can be used as pollutant indicators of small scale differences in air quality.

Lead image: Tree encrusted with common yellow lichen Xanthoria parietina