Changes of forest health in Norwegian boreal forests during 15 years

The Norwegian Monitoring Programme for Forest Damage has now been running since 1984. Its main objective has been monitoring the boreal forest conditions in relation to air pollution. Surveys of forests are performed on plots in a nation-wide representative grid network, in a network of local countywise plots, and in a network of intensively monitored plots within the framework of the internationally co-ordinated UN/ECE ICP Forests. At intensively monitored plots, a number of measurements were performed, for example tree crown-condition assessments, foliar chemistry, air pollution, precipitation chemistry (open area and stand throughfall), soil and soil-water chemistry. Vitality criteria have shown a declining trend, expressed as reduced crown density and more of discoloured trees, particularly in spruce forests. These results are similar to findings in other Scandinavian countries and coincide with the general trend in Europe. Forest yield has increased in later years in most parts of Norway. Tree mortality in excess of normal is not recorded. Considering these results, it is reasonable to presume that most Norwegian forest ecosystems are generally in a good condition. Areas in the south-eastern part of the country, however, may have shown indications of a slightly reduced crown condition. These areas are located in a region with a relatively high load of air pollution and low buffering capacity against acidification. Although forest conditions generally depend on soil, tree age, climate, pests and diseases, and other natural stressors; air pollution loads add to, or may interact with, these factors. Most likely inciting factors (e.g. summer drought) have occurred and produced visual symptoms. The actual effect of the air pollution component is, therefore, difficult to estimate; however, its importance is not discounted. (C) 2000 Elsevier Science B.V. All rights reserved.