Pinus banksiana seedlings were grown for 9 months in enclosures in greenhouses at CO2 concentrations of 350 or 750 μmol mol−1 with either low (0.005 to 0. 3 W m−2) or high (0.25 to 0. 90 W m−2) ultraviolet-B (UV-B) irradiances. Total seedling dry weight decreased with high UV treatment but was unaffected by CO2 enrichment. High UV treatment also shifted biomass partitioning in favor of leaf production. Both CO2 and UV treatments decreased the dark respiration rate and light compensation point. High UV light inhibited photosynthesis at 350 but not at 750 μmol mol−1 CO2 due to a UV induced increase in ribulose-1, 5-bisphosphate carboxylase/oxygenase efficiency and ribulose-1, 5-bisphosphate regeneration. Stomatal density was increased by high UV irradiance but was unchanged by CO2 enrichment.