Cuttings of a single birch clone (Betula pendula) were grown in field fumigation chambers throughout the growing season in either filtered air (control) or 90/40 nl O₃ l^–1 (day/night), both regimes being split into high and low nutrient supply. High nutrition was neither advantageous for maintaining the photosynthetic capacity and life span of the leaves (see Maurer et al. 1997) nor for limiting the productive loss of the whole plant under O₃ stress relative to low-fertilized (LF) plants. However, nutrition determined, through carbon allocation and leaf turn-over, the way plants coped with O₃ impact. High leaf turn-over under O₃ stress related the carbon gain of high-fertilized (HF) plants to the photosynthesis of newly formed, intact leaves, although the foliage area remained reduced (shedding of O₃-injured leaves, inhibited branching). In contrast, the low leaf turn-over of LF plants reflected the maintenance of the O₃-injured leaves, causing high respiratory costs in the whole-plant carbon balance and a root/shoot biomass ratio as low as in the HF plants. Within the root system, the carbon allocation was determined by nutrition rather than ozone, whereas the water-use efficiency of the whole-plant carbon increment was lowered by ozone in both nutrient regimes. The relationship between biomass production and nutrient levels in the whole plant was hardly affected by ozone, with only the range of interaction being narrowed. Conditions requiring the maintenance of foliage rather than favoring the replacement of O₃-injured leaves may render trees more susceptible to shifts in the carbon allocation.