The role clouds play in the modification of global radiation is still a major uncertainty in the risk assessment of UV effects on ecological systems and human health. This study presents cloud transmission data obtained from measurements with Robertson-Berger meters and simultaneous cloud observations. The global transmission of erythemally weighted irradiance depends strongly on cloud amount and can be described by a cubic function. The comparison with results derived from long-term records of total global irradiance indicates no statistically significant difference between the attenuation of erythemal and total global radiation. The large variance of data results from lumping together data from different cloud types. Classification of data according to cloud forms yields a more satisfactory fit. The coefficient of the cubic term characterizes the ability of various cloud forms to attenuate UV radiation. It varies between 0.4 for high clouds and approximately 1.0 for cumulonimbus. This attenuation parameter allows a quantitative description of the cloud influence on irradiance and therefore a more accurate risk assessment.

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Two clones of 5-year-old Norway spruce [Picea abies (L.)Karst.] were exposed to two atmospheric concentrations of CO2 (350 and 750/imol moP*) and O3 (20 and 75nmolmor') in a phytotron at the GSF-Forschungszentrum (Munich) over the course of a single season (April to October). The phytotron was programmed to recreate an artificial climate similar to that at a high elevation site in the Inner Bavarian Forest, and trees were grown in large containers of forest soil fertilized to achieve contrasting levels of potassium nutrition, designated wellfertilized or K-deflcient. Measurements of the rate of net CO2 assimilation were made on individual needle year age classes over the course of the season, chlorophyll fluorescence kinetics were recorded after approximately 23 weeks, and seasonal changes in non-structural carbohydrate composition of the current year's foliage were monitored. Ozone was found to have contrasting effects on the rate of net CO2 assimilation in different needle age classes. After c. 5 months of fumigation, elevated O3 increased (hy 33%) the rate of photosynthesis in the current year's needles. However, O3 depressed (by 30%) the photosynthetic rate of the previous year's needles throughout the period of exposure. Chlorophyll fluorescence measurements indicated that changes in photosystem II electron transport played no significant role in the effects of O3 on photosynthesis. The reasons for the contrasting effects of O3 on needles of different ages are discussed in the light of other recent flndings. Although O3 enhanced the rate at which CO2 was fixed in the current year's foliage, this was not reflected in increases in the non-structural carbohydrate content of the needles. The transfer of ambient CO2-grown trees to a CO2-enriched atmosphere resulted in marked stimulation in the photosynthetic rate of current and previous year's foliage. However, following expansion of the current year's growth, the photosynthetic rate of the previous year's foliage declined. The extent of photosynthetic adjustment in response to prolonged exposure to elevated CO2 depended upon the clone, providing evidence of intraspecific variation in the long-term response of photosynthesis to elevated CO2. The increase in photosynthesis induced by CO2 enrichment was associated with increased foliar concentrations of glucose, fructose and starch (but no change in sucrose) in the new growth. CO2 enrichment significantly enhanced the photosynthetic rate of K-deficient needles, but there was a strong CO2*soil interaction in the current year's needles, indicating that the long-term response of trees to a high CO2 environment may depend on soil fertility. Although the rate of photosynthesis and non-structural carbohydrate content of the new needles were increased in O3treated plants grown at higher levels of CO2, there was no evidence that elevated CO2 provided additional protection against O3 damage. Simultaneous exposure to elevated O3 modified the effects of elevated CO2 on needle photosynthesis and non-structural carbohydrate con...

SUMMARYTwo clones of 5-yr-old Norway spruce {Picea abies [L.] Karst.) were exposed to two atmospheric concentrations of CO2 (350 and 750/^.mol mol"^) and of O3 (20 and 75 nmol mol"^) in a phytotron at the GSFForschungszentrum (Munich) over the course of a single season (April-October). The phytotron was programmed to recreate an artificial climate similar to that at a high elevation site in the Inner Bavarian forest, and trees were grown in 40 1 containers of soil (pH 3-5) fertilized to achieve two levels of potassium nutrition; well fertilized and K-deficient. Foliar nutrient analyses performed at the beginning of the exposure indicated that the fertilization programmes achieved their goal without significantly altering the levels of other nutrients or the soil pH. At the beginning of the funiigation, foliar K concentrations were 7-9 mg g"^ d. wt for well fertilized trees and 4-5 mg g'^ d. wt for trees receiving no supplemental K. Over the course of the season, differences between K treatments intensified so that by the end of the experiment there was a five to sixfold difference between foliar K concentrations. This was associated with slight, but significant {P < 0-05), decreases in S and Zn (and of Cu in the 1989 needle year age class) and higher levels of C, N and Mg in K-deficient trees. Foliar N concentrations were low for all trees (9-15 mg g"^ needle d. wt) but were similar to levels found in the field.Elevated Og was found to decrease significantly the C (P < 0-05) and N (P < 0-001) content of both current-year (1989) and previous-year (1988) needles independent of CO.j concentration, but apart from some minor changes in the concentrations of Cu and Mn in the current-year needles no other effects of the pollutant on plant nutrient status were found. In contrast, CO, enrichment resulted in significantly (P < 0-01) lower concentrations of K and P (effects on Mg were also on the borderlines of statistical significance) in current-year needles, but there was no influence on the nutrient composition of the previous-year needles (although effects on N were on the borderlines of statistical significance). CO.^ enrichment also increased (P < 0-05) the C:N ratio of both current-year and previous-year needles. One factor contributing to the decline in foliar K at elevated COg appeared to be a marked increase (25-30%) in the rate at which cations were leached from the canopy by repeated simulated acid mist (pH 4-0) events, and this effect occurred independently of the Og concentration. The information presented will aid the interpretation of parallel studies examining the effects of elevated CO2 and/or O3 on seasonal changes in photosynthesis, non-structural carbohydrate content, antioxidants, tree growth and water use efficiency, and sheds further light on the growing scepticism concerning the role of O3 in the development of Mg and K-deficiency symptoms characteristic of certain types of forest decline in central Europe.