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In comparison with the effects of extended drought periods or severe nutrient stress, those of ozone are generally much milder, at least with respect to growth. However, there is substantial evidence from experiments, in the main using young saplings, that O $ does impose a stress on forest trees under European conditions. Decreased chlorophyll contents and photosynthetic rates, changes in carbon allocation, increased antioxidant activity, and reductions in biomass due to O $ have often been recorded, particularly in fast-growing species. Furthermore, O $ appears to weaken the trees' resilience to a range of biotic and abiotic stresses. Interactions between O $ and climatic stress, in particular drought and frost hardiness, are likely to result in potentially detrimental effects.A link between the occurrence of O $ and forest damage is not unequivocally established in Europe, and the problem remains of extrapolating and\or scaling up from studies on seedlings to predict responses to O $ of mature trees and forest stands, because we know so little about acclimation to O$ . An accurate assessment is also lacking of the magnitude of the O $ effect on European trees both in terms of the forest areas affected and its extent. In this review we suggest that C allocation is the key factor underlying the responses of trees to O $ . Stomata also play a key role, since the acquisition of C must be achieved while an effective control over water consumption is retained.Key words : Seedlings, mature trees, growth, drought, frost, ozone. Statistical evaluation of annual surveys of forest condition in Europe (UNECE, ICP Forests) over the last decade has shown that tree crown condition in the main damage areas cannot be wholly explained by the location, nature and climate of the site (Mu$ ller-Edzardz et al., 1997). This might imply an important role for external factors such as air pollutants in the decline of tree health. One quarter of the coniferous trees assessed were damaged ( 20 % defoliation) and damage was worst in central

SUMMARYRates of net photosynthesis were measured on Norway spruce, Picea abies (L.) Karst. exposed to three different concentrations of ozone for three seasons in open-top chambers. The treatments were non-filtered air + ozone (NF + ), non-filtered air (NF) and charcoal-filtered air (CF). Ozone was added daily between 11.00-18.00 local time. The study was performed during September and October, 1987. The rate of net photosynthesis was determined using an infrared gas analyser (IRGA) connected to a small cuvette, which was equipped with an artificial light source for unilateral illumination. The measurements were made on shoots of three different age classes: current year, 1 year old and 2 years old. Measurements took place at saturating light intensity (1150/^mol m"^ s^^), at a temperature of 16-6±0-7°C, and at a relative humidity of 67-5 + 5-3%. In current-year shoots the rate of net photosynthesis expressed on a needle-area basis was lowest in the CF treatment and highest in the NF treatment. In 1-and 2-year-old shoots, a decreased rate of the net photosynthetic assimilation rate, expressed on a needle area basis, was observed in trees from the NF and NF+ treatment compared with that of trees from the CF treatment. The effect increased with increasing ozone concentration and shoot age. A significant increase in the rate of respiration, expressed on a dry-weight basis, was observed in current year shoots from trees exposed to NF + . A significant decrease of the leaf conductance to carbon dioxide was obtamed m 2-year-old shoots exposed to NF and NF + . A decrease in the content of chlorophyll was observed in 1-and 2-year-old needles from trees exposed to NF+.

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SUMMARYOne clone of spruce, Picea abies (L.) Karst., was exposed to different concentrations of ozone in open-top chambers for three seasons. The treatments were filtered air (CF), non-filtered air (NF) and non-filtered air + ozone (NF + ). Changes in ultrastructure occurring in the 1985 flush of needles were followed for three years. Ultrastructural changes due to ozone first occurred in the chloroplasts. A decrease in length of the chloroplasts, an increase of the density of the stroma, an accumulation of ribosome-like granules and a decrease in the size of starch grains, were observed. The second organelle to be affected was the microbody and the last the mitochondria. At this later stage the cytoplasm had also disintegrated. Structural changes always appeared first in the outer cell layer facing the sky. The changes then gradually advanced in the tissue towards the lower part of the needle. In the NF+ treatment changes were observed at the first sampling event in November, 1 month after the end of the first exposure period. At this time the needles were about 6 months old. In the NF treatment the same changes appeared when the needles were about 14 months old. No changes were observed in the CF treatment. Changes of the same type as those observed in the mesophyll tissue were also seen in stomatal and endodermal cells. In albuminous cells a general breakdown of the organelles was observed.