Key message: Overyielding of stand biomass did not occur in a tree polyculture comprised of Betula pendula, Alnus glutinosa and Fagus sylvatica selected for contrasting traits. This was due to antagonistic interactions between the component species. Fine root dynamics and soil C stocks were unaffected by species mixture. Context: Increasing CO2 fixation in tree biomass through afforestation and forest management actions has potential for cost-effective climate mitigation. The influences of tree mixture on biomass production and subsequent soil C accumulation in polyculture still remain uncertain. Aims: We studied the polyculture of Alnus glutinosa (L.) Gaertn, Betula pendula Roth and Fagus sylvatica L. in a plantation forest to examine the effectiveness of species mixtures as a tool for increased biomass production and soil C accumulation. Methods: Tree biomass was estimated by developing species-specific allometric models and three years tree measurement. Fine root biomass and production were estimated by root coring and root-mesh methods. The ‘Relative Yield of Mixture’ approach was used to examine the mixture effect. Results: In mixture, an additive effect was observed in A. glutinosa (13% increase in basal diameter relative to the monoculture), however, there was no overall effect of mixture on total standing biomass due to the suppression of F. sylvatica (2.75 g m-2 reduction in woody biomass). Fine root biomass production showed no mixture effect. The quantity and quality of soil C (top 0.5 m) was not affected by tree mixture. Conclusions: We conclude that the contrasting growth responses of the A. glutinosa, B. pendula and F. sylvatica in polyculture resulted in no overyielding of standing biomass despite the complementary traits of the component species.