Lichens and air pollution

The number of tree-dwelling lichen species is rising once more, thanks to the reduction in emissions of acidifying substances. However, the emissions of ammonia are bringing about a shift in the species composition.

Trends

The gas sulphur dioxide (SO₂) has a toxic action on lichens, so in the 1960s and 1970s the large emissions of this gas from factories brought about a sharp decline in tree lichens in the Netherlands. During this period, tree lichens even totally disappeared in some areas.

In the period from 1979 to 2001 there was a strong increase in the number of species at the sampling points in Utrecht province. The picture was similar in other provinces. The most important reason for the recovery was a reduction in the emissions of SO₂. In Utrecht province approximately 85% of the lichen species increased their numbers.

Many of the species staging a comeback are nitrophytes; they occur particularly in areas with much intensive animal husbandry farming, where much ammonia (NH₃) is released into the atmosphere, from which it is deposited. The deposited ammonia greatly reduces the bark's acidity and increases its nitrogen content. This causes acidophilous species to decline, particularly those on oak and birch - tree species which traditionally harbour lichens.

Recent measurements (not shown on the graph) in Gelderland province (the Gelderse Vallei) show that having peaked in the 1990s, the nitrophilous species are now declining again. This could be the result of a decrease in the amount of ammonia.

Technical note

The figure shows the cumulative mean numbers of species per species group. Since the species differ in their sensitivity to SO₂, the number of species is a useful indicator of the effect of sulphur dioxide on lichens.

The data are from surveys conducted once every five years in Utrecht province, in which lichens on trees are measured at numerous sampling points.

References

• Herk, C.M. van (1998). Onderzoek naar de relatie tussen de ammoniakconcentratie en epifytische korstmossen. Lichenologisch Onderzoekbureau Nederland. Soest.
• Herk, C.M. van (2002). Monitoring van epifytische korstmossen in de provincie Utrecht 1979-2001. Lichenologisch Onderzoekbureau Nederland. Soest.