Current-year seedlings of beech, ash, Norway spruce and Scots pine were exposed during one growing season to different, but moderate, ozone (O₃) scenarios representative for Switzerland (50, 85, 100% ambient, 50% ambient+30 nl l⁻¹) in open-top chambers (OTCs) and to ambient O₃ concentrations in the field. Biomass significantly decreased with increasing O₃ dose in all species except for spruce. Losses of 25.5% (ash), 17.4% (beech), 9.9% (Scots pine) were found per 10 μl l⁻¹ h accumulated O₃ exposure over a threshold concentration of 40 nl l⁻¹ during daylight hours (AOT40). Ratios of root/shoot biomass (RSR) also significantly decreased with increasing AOT40 levels in beech and ash, but not in Norway spruce and Scots pine. The data show that the deciduous species beech and ash were more susceptible to O₃ with respect to RSR and biomass than the coniferous species Norway spruce and Scots pine.