DON HINRICHSEN is Acting Editor of Ambio, the International Journal of Human Environment, published by the Royal Swedish Academy of Sciences.
THE REMAINS OF A CANADIAN HARDWOOD FOREST effects from a nearby smelter
While it is generally agreed that acid rain kills fish and damages aquatic ecosystems, its effects on forests and soils are still being debated. Even more uncertain are its precise economic consequences.
The 1982 Stockholm Conference on Acidification of the Environment, with such questions very much in mind, began with a series of high-level meetings attended by 103 experts from 20 countries and four international organizations. The consensus that emerged among scientists after four days of discussion was a surprisingly detailed assessment of the current ""state of the art" of acid deposition and its environmental consequences.
There was also a thorough review of strategies and methods to control sulphur and nitrogen oxide emissions. The most startling evidence presented concerned the effects of acid deposition on forests and soils. The final report stated: "Concentrations of sulphur dioxide (SO2) considered to cause damage are smaller than was previously thought because physiological and biochemical changes of significance to growth may occur without the development of visible damage." The experts who prepared the report also agreed that there is evidence to suggest that tree growth may be decreased in association with annual mean concentrations as small as 25-50 m g of SO2.
Biochemist Bernhard Ulrich from the Federal Republic of Germany presented data indicating that about 1000000 ha of evergreen forests in Europe are adversely affected by acid deposition and that 10 percent of these are already dying, mostly in the Federal Republic of Germany. The life span of these evergreen forests may be reduced to a mere 30-50 years because of increasing acidification.
However, it is the indirect effects which may be the most insidious in the long run and the most difficult to quantify. The meeting drew the following conclusions about these effects:
· While SO2 might compensate for sulphur deficiencies in some forests, particularly those in tropical ecosystems, it is not thought to be a significant factor in temperate and boreal forests;
· Increased soil acidification will result in net losses of available magnesium, calcium and potassium, all essential elements for plant growth. This could very well decrease forest productivity;
· In areas of intense acidic deposition, such as in much of Scandinavia, losses of essential soil nutrients are aggravated by whole-tree harvesting methods and may jeopardize the ability of commercial forests to sustain yields. In many parts of central Europe there is already evidence that magnesium deficiencies have appeared;
· Acid deposition leaches heavy metals from soils, thereby increasing their toxicity. This toxicity is then transferred to plants and trees. Concern was expressed that in sensitive soils (such as silty or sandy soils overlying non-calcareous bedrock) with heavy amounts of acid deposition, the ratio of calcium to aluminium may be decreasing to the extent that root growth is impaired.
The report also noted that the uptake by plants of toxic elements in the soil, such as lead, aluminium, cadmium, zinc and manganese, may accumulate in plants which are then eaten by human beings and wildlife.
The situation was summed up by Ellis Cowling, chairman of the National Atmospheric Deposition Program of the United States. "In various regions of North America," he said, "economic damage to forests and agricultural crops is occurring as the result of dry deposition of toxic gases. Ambient concentrations of ozone, sulphur dioxide, and oxides of nitrogen, acting alone or in combination, are causing important damage and decreases in yield of both crops and forests."
This was supported by Lars Overrein, Director-General of the Norwegian Institute for Water Research, who warned of the long-term effects of heavy metals in combination with organic micropollutants in acid rain.
50 MILLION TONNES OF SO2 DISPERSE ACROSS EUROPE ANNUALLY wind currents still obey the laws of nature
The meeting report concluded that 50 to 60 million tonnes of sulphur dioxide, together with perhaps six million tonnes of nitrogen, are dispersed in the air across Europe every year.
The difference between the conclusions formed at the meeting as compared to those in the past was that, for the first time, experts in this field attempted to delineate a danger zone for sulphur dioxide concentrations. It was found that lakes in sensitive regions in Sweden, Norway, Canada and the United States do not suffer from long-term acidification if the sulphur deposition is 0.5 g/m2/yr or less. Above this level, damaging effects have been recorded, yet there is no single precise concentration level that can be used to indicate when the damage begins. Many scientists at the Stockholm Conference, however, were quick to counter this conclusion by stating that visible damage from acid rain does not begin to appear until the sulphur deposition rates reach 1.5 g/m2/yr. Whether damage occurs between 0.5 and 1.5 g/m2/yr is uncertain.
The final scientific report had been intended to serve as the basis for political discussion in a three-day ministerial conference that followed the expert meetings. In the end, however, the 40-page report was largely passed over by the ministers and other government representatives present.