Forests are in a good state of health

One of the most significant airborne nuisance factors affecting trees is acid fallout. It acidifies the soil and causes aluminium, among other substances, to dissolve in the soil, which makes it more difficult for trees to acquire nutrients. Among airborne pollutants acid fallout is caused by sulphur and heavy metals, such as cadmium, vanadium and lead.

By nature, forest soil will become somewhat more acidic in the long run, but acid fallout weakens the soil's capability of withstanding acidification. Of itself, forest soil possesses a very good capability of withstanding acidification.

Acidification can be prevented to some extent by such measures as prescribed burning, favouring broad-leaved trees in conifer forests, thinning excessively dense forests and final felling and regeneration of over-mature forests. None of these measures will eliminate airborne pollution, which can only be abolished by preventing polluting activity.

In Finland the defoliation of trees was the strongest at the end of the 1980s, after which the situation has improved. During the years 1986–95 the average defoliation of Scots pine was ten percent, which means that on an average, one tenth of the needles had disappeared when compared to a standard tree. For spruce, the corresponding figure was 22, and for deciduous trees, 10. However, the situation has further improved since. Airborne pollution has also gone down during the same period.

In particular, the contents of cadmium and vanadium in forest soil has gone down appreciably, by 50–60 percent from the level of 1985. The contents of copper and nickel have decreased by a generous tenth.

The greatest problem caused by acid fallout is nitrogen, for the volume of nitrogenous emissions is still not sufficiently low. Oxides of nitrogen are always generated in combustion reactions. On the other hand, nitrogen also acts as a nutrient, and a nitrogen deficiency often restricts the growth of forest vegetation. In the long run, the nitrogen load on the soil can also increase the leaching of nitrate into waterways.

According to studies made by the Finnish Forest Research Institute in 1998 and 2001, the state of health of the Finnish forests is good: fallouts of pollutants causing acidification have decreased, and on the other hand the effect of pollution on trees is less than was feared. Then again, the increase of nitrogen fallout during the previous century has accelerated forest growth.

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