Seedlings of Betula pubescens were grown at two CO 2 concentrations, in combination with either two O 3 concentrations or two air temperatures, during 34–35 days at 24 h day −1 photoperiod in growth chambers placed in a greenhouse. Increasing the CO 2 concentration from 350 to 560 μmol mol −1 at 17°C air temperature increased the dry weight of the main leaves, main stem, branches and root. The mean relative growth rate (RGR) was increased 10% by CO 2 enrichment, while increasing the O 3 concentration from 7 to 62 nmol mol −1 decreased the RGR by 9%. The relative biomass distribution between the different plant components was not significantly affected by the CO 2 concentration irrespective of the O 3 concentration. No significant interactions between CO 2 and O 3 concentration were found except on leaf size, which was stimulated more by elevated CO 2 concentration at high, compared to low, O 3 levels. In another experiment, elevated CO 2 (700 μmol mol −1) significantly increased the dry weight of the different plant components, and more at 20°C than at 15°C. Raising the CO 2 concentration increased the RGR by 5 and 10% at 15 and 20°C, respectively. CO 2 enrichment increased the branch dry weight relatively more than the dry weight of the other plant parts. Increasing the CO 2 concentration or temperature increased the plant height and stem diameter, however, no interactions between CO 2 and temperature were found.