In plants, the allocation of carbon to secondary metabolites has been shown to be determined by both the availability of resources (e.g., CO2 concentration) and by specific stress factors (e.g., ultraviolet [UV]-radiation). It has been suggested that, in combination, CO2 and UV-B radiation may differentially affect plant growth and morphogenic parameters, and elevated CO2 may ameliorate the effects of UV-B radiation. In the present work, the effects of increased atmospheric CO2 concentration and UV-B radiation on growth and the accumulation of different types of secondary metabolites were studied in silver birch (Betula pendula Roth). Seedlings were exposed to 350 and 700 μmol mol−1 of CO2 in a greenhouse. At both CO2 levels, additional UV-B was either present (8.16 kJ m−2 day−1 of biologically effective UV-B irradiance) or absent. The time course of accumulation of individual secondary compounds and the shifts in allocation of carbon between biomass and the secondary metabolites (phenolic acids, flavonoids, condensed tannins) were studied during a 1-month-long exposure. Additionally, the activities of enzymes ( l-phenylalanine ammonia-lyase [PAL], EC 188.8.131.52; peroxidase, EC 184.108.40.206; polyphenol oxidase, EC 220.127.116.11) were determined for leaves. UV-B radiation significantly increased biomass, PAL activity, and the accumulation of phenolic acids and flavonoids in seedlings. Elevated CO2 concentration increased the activities of all the enzymes studied and the accumulation of condensed tannins in leaves, especially with UV-B radiation. Because the observed UV-B induction of flavonoids was smaller under a high CO2 concentration, it was suggested that the excess of carbon in the atmosphere may moderate the effect of UV-B by increasing the metabolic activity of leaves (high enzyme activities) and by changing the allocation of internal carbon between different primary and secondary metabolites in the plant. Our results demonstrate the significant increase in the allocation of carbon to secondary metabolites without any large change in growth due to the elevation of CO2 concentration and UV-B radiation. There also was a stronger impact of CO2 than UV-B on the phenolic metabolism of birch seedlings.