Two clones of 5‐year‐old Norway spruce [Picea abies (L.) Karst.] were exposed to two atmospheric concentrations of CO2 (350 and 750 μmol mol−1) and O3 (20 and 75nmolmol−1) in a phytotron at the GSF‐Forschung‐szentrum (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 well‐fertilized or K‐deficient. 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 (by 33%) the rate of photosynthesis in the current year's needles. However, O3 depressed (by 30%) the photo‐synthetic 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 findings. 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 CO2soil 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 O3‐treated 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 content, emphasizing the need to take into account not only soil nutrient status but also the impact of concurrent increases in photochemical oxidant pollution in any serious consideration of the effects of climate change on plant production. Copyright © 1995, Wiley Blackwell. All rights reserved