Opinions concerning the usefulness of stand thinning at localities exposed to air pollution stress are not uniform. One of the reasons is that it has not yet been clearly demonstrated of the positive effect of improvement thinning of affected trees is dominant, or on the other hand if the negative effect induced by the greater exposure to air pollutants in the forest stand with loosened canopy plays its role. The problem is to elucidate ecophysical processes which take place in endangered stands during their thinning. An experiment was conducted to try to solve this problem: development of two young spruce stands was investigated for ten years at localities with the medium degree of air pollution (30 to 40 g per m3); the first stand had the full canopy (A), the other the loose canopy (B). Increased irradiance, mainly in the lower storey of tree crowns, was observed in the 13-year-old young stand where the tree density was reduced from 7 thousand trees per ha to 2,5 thousand trees per ha. The hgher intake of short-waved photosynthetically active radiation had very positive effects on the quantity and quality of assimilating organs. Differences in the temperature regime of stand profile were found out at the same time. The loosening of crown canopy weakened the compactness of the active surface of stand and decreased the extreme temperatures in the upper storey of crown space. The risks following from the synergism of pollutants and extreme temperatures are reduced by the loose canopy. Significant differences were also observed in the air flowin the crown space and in the pollution stress because the wind speed and pollution stress in loosened crowns increased two or three times. The higher photosynthetic activity of loose crowns and the higher pollution stress resulted in the higher content of sulphur in the two-year-old needles: the increases made 24% in young stands and 12% in the small pole stage. In spite of this increase neither sings damage in assimilating organs, nor decrease of b.h. increment were observed; on the contrary the basal area increment in loosened canopy was 45% larger. No differences with respect to the pollution-ecological significance were observed in the other factors of forest environment: these are soil temperature, water regime, rainfall sum under tree crowns, rain acidity.
