The total growth of Finnish forests has increased by more than 40 per cent since the beginning of the 1950s. The main reasons for an increase of such magnitude are to be found in changes in forest structure and silvicultural measures. The study of the effects of possibly improved soil productivity presupposes accurate analyses on stand and tree basis.
The forests close to the timber line are best suited for trend analysis. In the North the reactions to changing environment are strong while the effect of competition is weak. The trends found in the North are thus possibly indications of changes in the climate, in the concentration of CO2 in the atmosphere or of air pollution.
In the North, close to the Arctic Circle, both negative and positive growth trends have been measured. Local forest growth decline of unusual extent has been observed around non-ferrous smelters in the Kola Peninsula, North Western Russia. In addition to the total dieback of forests near the smelters, reduced growth of Scots pine has been detected up to 3035 kilometers from the sources of pollution.
Local forest decline covers only a small fraction of the forests around pollution sources. In the major part of forests in Lapland, the radial increment of Scots pine this century has been at a clearly higher level than in the 19th century. This is at least partly due to more favourable climatic conditions. The decreased competition between surviving trees after the late summer frost in 1902 accelerated the growth increase during the first decades of this century.
In southern Finland and Russian Karelia, forest growth trends are more difficult to detect because of the impact of forest management and higher competition between trees in dense stands. The dynamics of mixed forests, long-term cyclic variation of tree growth, effects of nitrogen deposition and of increased CO2 on soil productivity are also discussed in this paper.
Key words: growth trend, forest decline, nitrogen deposition.
Correspondence: Kari Mielikäinen, Finnish Forest Research Institute, PL 18, FIN-01301 Vantaa, Finland
Telefax: +358-0-8570 5361
E-mail: kari.mielikainen@metla.fi