Sulphur Dioxide :: Dwarf Shrub Heath

Impact Type: Direct exposure to pollutant

Key Concerns:

Relatively little information is available on exposure effects.

Key concerns are:

  • Visible decline symptoms for example, leaf discoloration.
  • Stimulated growth at low concentrations of S potentially changing community composition.
  • The vulnerability to direct damage of mosses, liverworts and lichens which are sensitive to lower concentrations than those causing injury to higher plants.

Grass spp. have been shown to evolve tolerance to SO2 in a short period of time at polluted sites. This however has led to reduced growth in clean air compared with sensitive genotypes (UKCLAG 1996).

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:

Effects would only be expected in the UK where lowland heathland occurs in the immediate vicinity of major sources of SO2.

See also Acid deposition :: Dwarf Shrub Heath

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).

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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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