Summary In a free-air carbon dioxide (CO2) enrichment study (BangorFACE), Alnus glutinosa, Betula pendula and Fagus sylvatica were planted in areas of one-, two- and three-species mixtures (n = 4). The trees were exposed to ambient or elevated CO2 (580 μmol mol−1) for 4 yr, and aboveground growth characteristics were measured. In monoculture, the mean effect of CO2 enrichment on aboveground woody biomass was + 29, + 22 and + 16% for A. glutinosa, F. sylvatica and B. pendula, respectively. When the same species were grown in polyculture, the response to CO2 switched to + 10, + 7 and 0% for A. glutinosa, B. pendula and F. sylvatica, respectively. In ambient atmosphere, our species grown in polyculture increased aboveground woody biomass from 12.9 ± 1.4 to 18.9 ± 1.0 kg m−2, whereas, in an elevated CO2 atmosphere, aboveground woody biomass increased from 15.2 ± 0.6 to 20.2 ± 0.6 kg m−2. The overyielding effect of polyculture was smaller (+ 7%) in elevated CO2 than in an ambient atmosphere (+ 18%). Our results show that the aboveground response to elevated CO2 is affected significantly by intra- and interspecific competition, and that the elevated CO2 response may be reduced in forest communities comprising tree species with contrasting functional traits.