Sulphur Dioxide :: Coniferous 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 needle 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.

The lichen flora of native pine woodlands in Scotland 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.

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:                   

The critical load for exposure to SO2 is set to a smaller value in winter, recognising the increased susceptibility of vegetation under these conditions (Makela et al. 1987).

Factors controlling the distribution of sensitive epiphytic lichen species need to be reassessed following reductions in SO2 concentrations, to address rates of recovery and recolonization earlier distributions.

Dry deposited S has been demonstrated to interact more closely with Scots Pine foliage than wet deposited S (Manninen et al,1996), emphasising the role of direct SO2 effects. Urban amenity woodlands are exposed to the largest SO2 concentrations from dry deposition and therefore more likely to exhibit visible foliar damage. Substantial decreases in urban SO2 levels (due to pollution control since 1980) are leading to a substantial reduction in exposure (RoTAP 2012).

Evidence of a synergistic interaction between SO2 and the green spruce aphid (Elatobium abietinium) has been shown for on various growth parameters of Sitka spruce (DOE 1993).

Shortage of suitable experimental and observational data is a problem. Most available data is for young trees and saplings only.

See also Acid deposition :: Coniferous 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.



Ashmore, M.R.; Wilson, [Eds.] R.B. 1994 Critical levels of air pollutants for Europe Report of the Egham workshop
Gries, C. 2008 Lichen sensitivity to air pollution Lichen Biology
Makela, A.; Materna, J.; Schoop, W. 1987 Direct Effects of Sulphur on Forests in Europe - a Regional Model of Risk International Institute for Applied Systems Analysis (WP-87-57)
Manninen, S.; Huttunen, S.; Rautio, P.; Peramaki, P. 1996 Assessing the critical load of SO2 for Scots Pine in situ. Environmental Pollution 93 27-38