Defoliation of coniferous trees-assessments 1984–87

Information about visible symptoms of forest damage was collected during 1984–1987 from 13 reference areas established by the Swedish Environmental Monitoring Programme. The annual occurence of needle loss on Norway spruce and Scots pine was estimated visually and expressed as a percentage of defoliation on the upper parts of the tree-crowns. As the reference areas are situated all over Sweden from Lat. 55° to 69° the climate factors, such as temperature, winds, drought, humidity and snow cover, vary within a wide range. There are also altitudinal differences-sites are situated from 50 to 720 m above sea level. Two of the northernmost sites, which are also the high altitude ones, had the highest defoliation values. In the south, the most severely defoliated trees were growing on sites near the western and the eastern coasts. The estimated defoliation level was mainly lower in 1986 and 1987, compared with the level in 1984. A wide range of environmental factors influence tree health. Trees are subjected to several kinds of stress-factors causing several kinds of damage. Besides the natural stresses, there is also, a long-term effect of air pollutants. Diversity in resistance and susceptibility to air pollutants, as well as the exposition to winds and humidity, are affecting needle loss.The exposures to chemicals occurring under forest conditions are different at different parts of Sweden. The observed defoliation of conifers does not seem to have a linear relationship with the pollution gradient but are also attributable to natural stress factors.