Several investigations about air pollution impact on forests have been carried out in Slovenia during last thirty years, mainly using the Norway spruce (Picea abies [L.] Karst.) as the main bioindication species, but there is a lack of information about common beach (Fagus sylvatica L.) condition, the most important forest tree species in Slovenia and in Zasavje area. The assessment of air pollution effect on beech based on stress physiological indicators of foliage, macronutrients level in beech leaves and by assessment of crown defoliation was carried out altogether with necessary soils analyses of the selected beech stands. On the basis of all parameters it can be concluded that the worst condition of beech trees in Zasavje is due to high level of air pollution, mainly by sulphur dioxide and not by deficiency of macronutrients or unfavourable state of soils

The analysis of the latest results along with the change analysis reveals that the conditions of Slovenia's forest are considerably stable and that the process of decline has not progressed with the intensity, that had been predicted in the early eighties. Quite favourable conditions are due to prompt actions (selection and sanitary cutting) in the field

Determination of the streses operating on a forest ecosystem demands the use of several bioindication methods. Air quality indicators were assessed from an inventory of forest decline based on the assessment of tree crowns and lichens. Photosynthetic pigments, ascorbic acid and major macro nutrients were studied in mountain pine (Pinus mugo Yurra) needles as indicators of physiological and biochemical stress. Analyses were carried out at selected forest plots and transects in predominantly unpolluted areas within Triglav National Park, Julian Alps, Slovenia. It was assumed that air pollutant input in forest ecosystems is of two kind, e.g. local from the air pollutin sources in the valleys on the North-West part of tha national park and transboundary source at high altitudes