This paper describes a relationship between ozone exposure, biomass, visual symptoms and a chlorophyll a fluorescence performance index for young beech trees (Fagus sylvatica). The plants were exposed to four levels of ozone in open-top fumigation chambers (50, 85, 100% of ambient, and 50% of ambient+30 nl l⁻¹ ozone) that fluctuated in parallel with ambient ozone during a single growing season. The trees were fumigated in the four treatments with ozone levels corresponding to an AOT40 (accumulated exposure above a threshold of 40 nl l⁻¹) of 0.01, 3.35, 7.06 and 19.70 μl l⁻¹ h, respectively. Highly significant differences were found between the 50% of ambient+30 nl l⁻¹ ozone treatment and all other treatments, with a 70.5% reduction in primary photosynthetic performance, as measured with the PI index. The reduction of the PI values demonstrated a high correlation with visual symptom development (r²=0.98), and by the end of September with biomass loss (r²=0.99). A significant ozone exposure–response relationship was found between AOT40 and primary photochemistry (r²=0.97). Thus, analysis of PI provides an alternative method for regional monitoring of tree health within the context of the currently employed AOT40.