We constructed a carbon budget for young birch trees grown in ambient and elevated CO2 concentrations over their fourth year of growth. The annual total of net leaf photosynthesis was 110% more in elevated CO2 than in ambient CO2. However, the trees in elevated CO2 grew only 59% more biomass than the trees in ambient CO2 over the year. Modelling studies showed that larger loss of carbon from fine-root production and growth of the root-associated mycorrhiza by the trees in elevated CO2 probably accounted for all the remaining difference in net photosynthesis between the two treatments. Our modelling also showed that the fraction of net photosynthate consumed by respiration of nonleaf tissue was similar in the two CO2 treatments, and was 26% and 24% for trees in ambient and elevated CO2, respectively.
Trees in elevated CO2 had 43% more leaves, and produced 110% more net photosynthate than trees in ambient CO2, even though the maximum rate of carboxylation per unit leaf nitrogen decreased by 21%. Sensitivity studies showed that down-regulation reduced the annual net photosynthetic production of the trees in elevated CO2 by only 6%. Direct effects of higher CO2 on photosynthesis and greater leaf area of the trees in elevated CO2 increased the net photosynthesis of the trees by 68% and 60%, respectively; and together accounted for most of the difference in net photosynthesis between the two treatments.