SO2 :: Lichens (general)

Latin name: 

Lichens (general)

Impact Type: Direct exposure to pollutant

Key Concerns:

It has long been known that SO2 has an effect on lichen growth, maps using the decline of lichens as a system for the measuring SO2 levels were available before the First World War (Dudley & Stolton 1996).

Studies suggest that atmospheric SO2 and NO2 are the two most important factors in determining levels of lichen biodiversity (Hawksworth & Rose 1970a,b, Richardson 1988, van Dobben et al. 2001). However it should be noted that not all lichen species are affected by exposure to SO2 and some species are more tolerant that others. For example, Lecanora conizeoides is tolerant of and can benefit from the presence of some SO2 (van Dobben et al. 2001). Consequently L. conizeoides may be in decline in some areas following reduction in SO2 levels (Bates et al. 1990). It has also been found that species which are sensitive to one form air pollution, are not automatically sensitive to others (van Dobben & ter Braak 1998, 1999, van Dobben et al. 2001).

Additional Comments:

In the late 1960's Hawksworth & Rose (1970a,b) developed a ten point scale for predicting levels of SO2 in the air in England and Wales using the lichens growing on trees which is still in use. However, recent analysis (Bates 2001, van Dobben et al. 2001, van Dobben 1996) suggest that this scale now need to be revised downward to reflect a changing pollution climate.

Hawksworth & McManus (1998) have reported lichen recolonization in London corresponding to the decline of SO2. They report finding 25 species which had not been recorded within 16 km of the centre of London in the 20th century, 8 of which had not been seen during the previous 200 years. A similar pattern was observed in Finland over 20 years from 1980 (Ranta, 2001). The role of lichens as bioindicators of SO2 pollution has been reviewed by Nash & Gries (2002).

Critical levels have been set according to different species groups (for more detail see Ssp. groups, e.g. Lobaria spp.), based on the Hawksworth & Rose scale (Hawksworth & Rose 1970a,b), but this is currently under review as critical levels may be smaller than previously estimated. 

Critical Load/level: 

Habitat/ Ecosystem Type Critical Load/ Level Status Reliability Indication of exceedance Reference
Forests and semi-natural vegetation

20 µg SO2 m-3 annual mean and half-year(Oct-March) mean

UNECE, 2004 quite reliable i.e. the results of some studies are comparable

Low temperature appears to enhance the negative effects of SO2, and the lower critical level of 15 µg SO2 m-3 is used where the effective temperature sum (ETS) (i.e. the sum of temperatures) above 5oC is below 1000 degree days (d.d) (Ashmore et al., 1994).



Hawksworth, D.L.; Rose, F. 1970 Lichens as Pollution Monitors Studies in Biology (66)
Hawksworth, D.L.; McManus, P.M. 1989 Lichen Recolonization in London under Conditions of Rapidly Falling Sulfur-Dioxide Levels, and the Concept of Zone Skipping. Botanical Journal of the Linnean Society 100 99-109
Nash, T.H.; Gries, C. 2002 Lichens as bioindicators of sulfur dioxide Symbiosis 33(1) 1-21
Richardson, D.H.S. 1988 Understanding the Pollution Sensitivity of Lichens Botanical Journal of the Linnean Society 96 31-43
Van Dobben, H.J.; Ter Braak, C.J.F. 1998 Effects of atmospheric NH3 on epiphytic lichens in the Netherlands: the pitfalls of biological monitoring. Atmospheric Environment 32 551-557
Van Dobben, H.J.; Ter Braak, C.J.F.; Dirske, G.M. 1999 Undergrowth as a biomonitor for deposition of N and acidity in pine forest. Forest Ecology and Management 114 83-95
Van Dobben, H.J.; Wolterbeek, H.T.; Wamelink, G.W.W.; Ter Braak, C.J.F. 2001 Relationship between epiphytic lichens, trace elements and gaseous atmospheric pollutants Environmental Pollution 112 163-169

Species group: