Elevated CO2 enrichment induces a differential biomass response in a mixed species temperate forest plantation

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 (580molmol1) for 4yr, 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 +/- 1.0kgm2, whereas, in an elevated CO2 atmosphere, aboveground woody biomass increased from 15.2 +/- 0.6 to 20.2 +/- 0.6kgm2. 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