Article (refereed) -postprint Elevated Co 2 Enrichment Induces a Differential Biomass Response in a Mixed 1 Species Temperate Forest Plantation 2 3

Elevated CO2 enrichment induces a differential biomass response in a mixed species temperate forest plantation. The NERC and CEH trademarks and logos ('the Trademarks') are registered trademarks of NERC in the UK and other countries, and may not be used without the prior written consent of the Trademark owner. Acknowledgements: 89 words 29 Number of Figures: 6 30 Number of Tables: 7 31 32 33 Summary 34  In a free-air CO 2 enrichment study (BangorFACE) Alnus glutinosa, Betula 35 pendula and Fagus sylvatica were planted in areas of one, two and three 36 species mixtures (n=4). The trees were exposed to ambient or elevated CO 2 37 (580 µmol mol-1) for four years, and aboveground growth characteristics 38 measured. 39  In monoculture, the mean effect of CO 2 enrichment on aboveground woody 40 biomass was +29, +22 and +16% for A. glutinosa, F. sylvatica, and B. pendula 41 respectively. When the same species were grown in polyculture, the response 42 to CO 2 switched to +10, +7 and 0%, for A. glutinosa, B. pendula, and F. 43 sylvatica respectively. 44  In ambient atmosphere our species grown in polyculture increased 45 aboveground woody biomass from 12.9 ± 1.4 kg m-2 to 18.9 ± 1.0 kg m-2 , 46 whereas in an elevated CO 2 atmosphere aboveground woody biomass 47 increased from 15.2 ± 0.6 kg m-2 to 20.2 ± 0.6 kg m-2. The overyielding effect 48 of polyculture was smaller (+7%) in elevated CO 2 than in an ambient 49 atmosphere (+18%). 50  Our results show that the aboveground response to elevated CO 2 is 51 significantly affected by intra-and inter-specific competition, and that 52 elevated CO 2 response may be reduced in forest communities comprised of 53 tree species with contrasting functional traits. 54 keywords: 55 Free-air CO 2 enrichment (FACE), temperate forest, alder (Alnus gl u tinosa), silver 56