Boreal and temperate forests are not necessarily declining in total extent, but they are subject to major changes in species composition and local distribution. Before dying, forests show signs of ill health, such as sparse foliage, leaf discolouration, undersized foliage, leafless twigs and dead branches. There is accumulating evidence that the many recorded instances of mass tree deaths throughout the temperate and boreal forests of the northern industrialized hemisphere must be linked to a common cause. Whatever the final trigger mechanism (pests, diseases, drought), the tree population in many of these forests has been reduced to a susceptible condition because it has passed a threshold of tolerance to sustained attack from chemical pollutants. Although different experts claim predominance for one mechanism or another, there is little in the explanations which is mutually contradictory. Deaths and decline in many different species of tree are now so widespread over such a variety of sites among stands of different ages and at different altitudes that no single hypothesis can suffice to explain every instance. The most probable explanation remains a combination of mechanisms in different local circumstances, but induced by growth stress from excess nitrogen. Together these mechanisms add up to an overwhelming stress burden upon the tree population which is only just beginning to show its effects and will require considerable social adjustment to reverse.
Trees are affected in various ways in Europe. There are such natural causes as drought in the case of maritime pine and insects in the case of oak trees. Other causes include human activities and their effects, starting with air pollution. The pollutants include sulphur, for long the main culprit, and still dominant in central and eastern Europe. In western Europe it has been overtaken by nitrogen, which is emitted by motor vehicles, certain industries and industrial livestock farming. Ozone is harmful, too. Acid forest soil means another danger.
Recent evidence indicates that forests throughout the eastern USA are in decline, perhaps seriously. Man-made pollution is the chief suspect. Some of the symptoms are similar to those that eventually led to a dramatic decline in central European forests. Research shows that some species of softwood trees are losing their foliage, dying and failing to reproduce at high elevations in some parts of the Appalachian Mountains. Some studies show a large-scale, rapid and simultaneous drop in the growth rates of a half dozen species of coniferous trees in the east. This trend started around 1960 and has apparently accelerated over the past 10 years. Some hardwood trees are also showing these symptoms, although to a lesser degree.
In 1990 it was estimated that at least 7 million hectares of forest in 15 European countries were suffering from Waldsterben, death of the forest. One third of Swiss forests are suffering moderate damage, 10% are severely affected, and 2% are dead. Another example of the extent of the damage is Germany, where official studies show that up to half the country's trees, which cover a third of its area, are either diseased or dead because of airborne industrial pollution and vehicle emissions. For British trees, this figure is 60%, for Czechoslovakia 41% and for France 7%.
In 1991 the UN Economic Commission for Europe estimated that the productivity of European forests is declining at US$30 billion a year. In a single lifetime, an area the size of Europe in the USA was deforested. Approximately 85% of the primary, old-growth forests of the U.S. were destroyed by the original European settlers." Redwood and sequoia forests that once blanketed the coastal ranges of the USA have been reduced to sparse remnants amounting to only 4 percent of their original extent." Siberia may be losing 4 million ha of forest per year, twice the deforestation rate of Brazil.
About 60 per cent of all forests in Western and Central Europe, and large areas around industrial installations in East Europe and Central Asia, are either seriously or moderately degraded, mainly as a result of pollution. In some areas of Europe, however, there have been some improvements in forest condition, which are being interpreted as a response to improvements in air quality (UNECE/CEC 1997).
In the European Union (EU) only 54.8% of trees, of all species, were classified as "unhealthy" in 1998. Among them, 37.1% showed signs of slight defoliation, 17% were seriously damaged, having lost 25% of their leaves or needles, while 0.7% had died. The situation appeared to be worse in Europe as a whole, given that the most seriously affected forests are to be found in the eastern half of Europe, outside the frontiers of the present EU. Trees and forests suffered damage particularly in Europe and Italy. Broad-leaved trees, such as beech and several species of oak, are deteriorating more than the others. Maritime pines are also in an alarming condition. The areas showing improvement are located in Germany and Poland above all, with the Nordic countries recording no change. Scots pines are recovering in eastern Germany and Poland, as is the holm oak in certain region.