The effects of increased ultraviolet-B (UV-B) radiation on the growth, mycorrhizas and mineral nutrition of silver birch (Betula pendula Roth) seedlings were studied in greenhouse conditions. Seedlings—planted in a birch-forest top soil and sand substrate—were grown without additional nutrient supply. Ultraviolet treatment started immediately after the seedlings emerged and the daily integrated biologically effective UV-B irradiance on the UV-B-treated plants was equivalent to a 25% depletion of stratospheric ozone under clear sky conditions. Visible symptoms of UV-B damage or nutrient deficiency were not observed throughout the experiment. Seedling height and dry weight (DW) (measured after 58 days and 76 days of treatment) were not affected by increased UV-B. However, a significant shift in DW allocation toward roots resulted in a lower shoot/root ratio and leaf area ratio in UV-B-treated plants compared to control plants. At the first harvest (after 58 days of treatment), the percentage of various mycorrhizal morphotypes and the number of short roots per unit of root length or weight were not affected by increased UV-B despite significantly increased DW allocation toward roots. Subtle reduction in the allocation of nitrogen (N) to leaves and increased allocation of phosphorus (P) to roots may suggest cumulative effects that could affect the plant performance over the long-term.