The effects of supplementary far-red sidelight on the formation of mycorrhizas and on the accumulation and allocation of dry weight and mineral nutrients were studied in Scots pine (Pinus sylvestris L.) seedlings. Starting one week after germination the seedlings were subjected to two different light quality regimes: control and simulated sparse-canopy conditions (FR+). In the FR+ regime, light reflected by neighbouring plants was simulated by means of supplementary far-red light sources, which reduced the horizontal red/far-red photon ratio (R:FR) without affecting PAR. Seedlings were harvested after three months of treatment. FR+ increased stem height and decreased the total dry weight of seedlings. Dry weight allocation to needles was not affected, whereas dry weight allocation to roots was reduced and that to stems was increased in FR+ treated seedlings. The total number of short root tips and developing mycorrhizas per seedling were lower in FR+ than in control plants. Most short roots were developing mycorrhizas, while non-mycorrhizal short roots and mycorrhizas with mantle or external mycelium were very scarce. Changes in the allocation of nutrients in general followed the changes in dry weight allocation, and changes in nutrient content followed those in total dry weight. However, mismatches among these changes resulted in significant changes in nutrient concentrations in some organs: the concentrations of nitrogen and potassium in needles and the concentration of nitrogen in stems were higher in FR+ than in control seedlings. Changes in biomass and nutrient allocation under low R:FR may promote rapid height growth during early development in stands of Scots pine seedlings, but concomitant reductions in growth of the root system and mycorrhizas may negatively affect tree performance over the long term.