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Forest threats

Female bark beetle boring tunnel in which she will lay her eggs. (in new window)

Female bark beetle boring tunnel in which she will lay her eggs.© WSL Birmensdorf

The bark beetle at work: the bark comes loose and the tree dies (in new window)

The bark beetle at work: the bark comes loose and the tree dies© WSL Birmensdorf

About one quarter of all trees in European forest are in some way damaged, according to a UN report issued in 2001. Since neither pests nor pollution nor weather conditions stop at borders, Switzerland's trees have not been spared. Some of the damage will reverse itself naturally - insect attacks, devastating as they are, usually go in cycles.

Bark beetle

A tiny beetle, about 5 mm long, (2/10th inch), wrought havoc in Swiss forests in the aftermath of the 1999 hurricane Lothar. The presence of the bark beetle is nothing unusual: in normal circumstances it lays its eggs in dead wood - mainly spruce - and feeds on weak or dying trees which it attacks by burrowing under the bark, and its numbers are restricted by predators. However, the thousands of trees uprooted by Lothar provided such a favourable breeding ground that it produced the worst infestation for 200 years. The beetle spread to healthy trees, and once a tree is attacked there is no known way to save it and it has to be felled.

Infestation remained high for several years, exacerbated by hot summers.

A similar attack by bark beetles followed hurricane Vivian in 1990; it peaked in 1992 and numbers subsequently returned to normal.

Larch bud moth (Zeiraphera diniana)

This moth has devastated larch forests in tourist resorts in the Engadine Valley in Canton Graubünden at regular 8-9 year cycles. The increase at the height of the cycle is mind-boggling: there are about 100,000 times more insects than there are at the trough.

The moth affects larch, stone pine and Norway spruce. Its caterpillars attack the young shoots, causing the trees to lose many of their leaves, with the result that photoynthesis - the process by which green plants take energy from the sun - cannot occur. As a result, growth is stunted. The summer forests turn red as if it is autumn.

This moth is incidentally of great interest to scientists because they have discovered genetic differences between the larch-feeding form and the form that feeds on pine, and they believe that the two are in the process of separating into two distinct species.

Threats from animals

Foresters have longed blamed deer for destroying silver fir saplings, and altering the composition of forests.

However, research carried out as part of the National Research Programme on landscapes and habitats in the Alps has shown that deer are only one factor among many. Of animals, more damage is caused by mice gnawing at saplings.

The researchers also found that given sufficient light, a sapling can recover completely despite having been gnawed.



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