What is ‘acid rain’?
‘Acid rain’ is a term often incorrectly used to refer to all types of air pollution caused by human activities.
Rainwater is naturally acidic as a result of carbon dioxide (CO2) gas (which is naturally present in the atmosphere), dissolved in water. Rainwater can become more acidic when man-made pollutants also become dissolved. This acidity is mainly derived from the washout of oxides of sulphur (SO2) and nitrogen (NO2). Although these compounds exist naturally in the environment, by-products of burning fuels such as coal and oil now form the largest source of man-made acidity in the atmosphere. Once in the atmosphere, these oxides can mix with other chemicals to form more harmful pollutants that either settle as ‘dry deposition’ or are washed out by rain and fall as ‘wet deposition’.
The most significant man made acids in precipitation are sulphuric (H2SO4) and nitric (HNO3) acid. In addition to acid pollutants entering the forest ecosystem through dry and wet deposition, water droplets in the form of cloud, fog or mist can be many times more acidic than rainwater and these are deposited on forest canopies, especially in upland areas (‘occult deposition’).
Most of the SO2 and NO2 produced in Britain come from power stations and large industrial units, but cars and heavy vehicles are also important sources of the oxides of nitrogen.
How can acid deposition affect the environment?
Acid deposition enters an ecosystem in a variety of forms and via a number of pathways. It may enter the soil directly or interact with the tree canopy as well as understory vegetation. Within the canopy, it may become more acidic as a result of the leaching of accumulated dry deposition or organic acid anions from the foliage. On the other hand, the acid precipitation may become less acidic through exchange processes of H+ for base cations in plant tissues. Up to 80% of the proton deposition may be buffered by the foliage. As a result of this, direct injury to the foliage has occurred close to point sources of pollution. Further away from the pollution source, indirect injury may occur including the acidification of soils, lakes and rivers. The direct effect of acid deposition on trees, as distinct from indirect effects acting through the soil are still unclear.
Sulphur deposition has fallen greatly over the past 20 years as a result of emissions control measures and it is now considered to be less of a threat to tree health than other pollutants such as ozone and ammonia. Since the 1940’s, atmospheric nitrogen pollution has steadily increased, primarily as a consequence of the combustion of fossil fuels and agricultural intensification. Agricultural ammonium emissions are becoming an increasingly important contributor to nitrogen deposition.
Nitrogen is an essential nutrient for plant growth. However, if nitrogen inputs to a forest ecosystem is exceeding both the biological demand and the storage capacity of the soil, then ‘nitrogen saturation’ is possible. This will potentially lead to nitrogen leaching from the soil, causing soil acidification as well as surface and groundwater contamination. The build up of nitrogenous compounds can have damaging effects on the wider environment in a process known as ‘eutrophication’. This can have major impacts on plant communities leading to changes in species composition and the sensitivity of vegetation to environmental stresses, such as drought, frost or insect predation.
Methods have been developed to set critical loads to protect against these adverse effects.
Does acid deposition affect forest ecosystems in Britain?
Long-term impacts research and monitoring of atmospheric pollution effects on the acidification of soils and tree vitality has shown that:
- Widespread damage to forests and trees has not been occurred as a result of atmospheric pollution.
- Significant areas of forest are, however, being affected by acid deposition according to the critical loads approach to ecosystem evaluation.
- Sulphur deposition has decreased, while nitrogen deposition has not (see Level II trends); nitrogen is now of most concern, both for its role in acidification and eutrophication.
- Nitrogen deposition may be leading to a change in the groundflora associated with woodland; this phenomenon is particularly associated with forest edges and the proximity of intensive agriculture.
- A recent survey of forest condition shows that both needle retention and the incidence of insect damage to Scots pine are related to nitrogen deposition.
- Soil acidification has historically occurred in the UK; however, evidence is limited which relates soil acidification to impacts on soil function and/or tree vitality.
- A survey of the soils in Level I forest condition plots found no link between above-ground forest condition or growth and soil acidification (see Level I plots).
- Manipulation experiments have established clear relationships between soil acidification and tree root functioning.
Future research will concentrate on below-ground cause-effect relationships which may be more sensitive to pollution.