Elevated CO 2 enrichment induces a differential biomass response in a mixed species temperate forest plantation

SummaryIn a free‐air carbon dioxide ( CO 2 ) enrichment study (Bangor FACE ), 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 CO 2 (580 μmol mol −1 ) for 4 yr, and aboveground growth characteristics were measured. In monoculture, the mean effect of CO 2 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 CO 2 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 CO 2 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 CO 2 than in an ambient atmosphere (+ 18%). Our results show that the aboveground response to elevated CO 2 is affected significantly by intra‐ and interspecific competition, and that the elevated CO 2 response may be reduced in forest communities comprising tree species with contrasting functional traits.