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Acidic Deposition--Soils and Forests

AUTHOR: Pamela Harter
DATE: July 1989
FULL TITLE: Acidic Deposition--Ecological Effects on Soils and Forests

Soil acidification and the widespread recent damage to forests are considered in this review. The evidence that environmental effects are occurring is examined to see if a trend of increasing acidification can be related to changes in atmospheric deposition of sulphates and nitrates. Possible mechanisms of damage are considered, to clarify the contributions of variations in natural stresses and other pollutants.

It is concluded that acidic deposition, originating partly from emissions of pollutant gases, arising from man-made sources including combustion of fossil fuels, contributes to natural processes in some regions. Acidification of unmanaged soils leads to enchanced leaching of nutrients, thus reducing their availability to plants. It also mobilises aluminum and other toxic metals in the soil, exposing plants to higher concentrations. Soil acidification can increase both the acidity and the concentration of metals in the drainage water entering lakes and streams.

The novel type of forest damage in central Europe has affected a wide variety of species over a wide range of stand and tree ages. Affected trees suffered a slowdown in growth up to 30 years ago, in a period of rising gaseous pollutant emissions. It seems clear that atmospheric pollutants, including those emitted from energy-related sources such as coal combustion, play some part in the novel type of forest damage. However, the mechanism of action is at present unproven. The type and degree of forest damage found in an area may be related to the overall pollution climate and soil conditions, rather than to the concentration level of a few particular pollutants. Climatic conditions may act as synchronizing factor leading to the appearance of damage.

The scope of the review is confined to the scientific evidence of ecological effects and thus excludes policy considerations and details of control strategies. The reference list includes only documents cited in this review.