Sulphur Dioxide :: Broadleaved, Mixed and Yew Woodland

Impact Type: Direct exposure to pollutant

Key Concerns:

Impacts of SO2 on tree health include:

  • Visible decline symptoms for example, abnormal branching patterns, reduced crown density and leaf discoloration.
  • Poor general tree health (Woodin and Farmer 1993).
  • Subtle changes in morphology, physiology and biochemistry can occur which do not affect tree growth but increase the sensitivity of trees to environmental factors such as wind, frost , drought and pests.

The most sensitive component is often the ephipytic lichen flora. A large number of foliose and fruticose lichens are particularly sensitive to SO2 exposure leading to the use of lichens as bioindicators for SO2.

Risk Areas

The concentrations of SO2 in the UK are well below the critical level with the largest concetrations in the urban and suburban areas of the country, with values of a few ppb, and in the rural Midlands and home counties, with concentrations of 1 to 2 ppb (2.7 - 5.4 ug  m-3). Elsewhere, in rural areas and especially in the west and north of the UK, SO2 concentrations are low at below 0.5 ppb (<1.3 ug SO2 m-3). Emissions from UK shipping are highest close to major ports where elevated concentrations may occur locally (ROTAP, 2012).

Additional Comments:

Deciduous woodlands have not been studied to the same extent as coniferous forests, although surveys have been carried out for beech (Ling et al. 1993).

The critical level for exposure to SO2 is set to a smaller value in winter, recognising the increased susceptibility of vegetation under these conditions.

Habitat Specific information

The lichen flora of Atlantic oak woodlands is of particular conservation value. At present species are absent from areas where SO2 concentrations have decreased and are now less than the critical level. This may indicate that recovery could take many years (Farmer et al. 1991b).

See also Acid deposition :: Broadleaved, Mixed and Yew Woodland

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).

Cyanobacterial lichens

10 µg SO2 m-3 annual mean

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

SO2 dissolves in water to produce acidic ions which are readily absorbed through the lichen thalli disrupting photosynthesis. SO2 has also been shown to inhibit the activity of nitrogenase, which is used by cyanobacterial photobionts to fix atmospheric nitrogen.

Reference: Gries, C. (2008). Lichen sensitivity to air pollution, Chapter 13 in Nash, TH, III (ed.) Lichen Biology (2nd. ed.), Cambridge University Press, Cambridge.


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