Effects of Elevated CO2 Concentrations on Leaf Senescence and Late-Season Net Photosynthesis of Red Maple (Acer rubrum)

To investigate the effects of elevated atmospheric CO2 concentrations ([CO2]) on autumnal phenology and end of season photosynthesis of different bud-break leaves of trees, we fumigated 2-year-old red maple seedlings with 800, 600, and 400 mu L L-1 [CO2] in nine continuous stirred tank reactor (CSTR) chambers. Leaves were subdivided into first (B1), second (B2), and third bud-break (B3) leaves. The results indicated that (1) autumnal leaf senescence, including the beginning date, end date, and duration of leaf abscission of all three bud-break leaf groups, was not affected by elevated [CO2]; (2) elevated [CO2] increased leaf photosynthesis of B1, B2, and B3 leaves throughout the whole of the growing season; (3) elevated [CO2] significantly increased whole plant photosynthesis only for B2 leaves, accounting for 41.2-54.7% of the whole plant photosynthesis, due to the larger whole leaf area of B2. In conclusion, enhanced seasonal carbon gain in response to atmospheric CO2 enrichment is the result of strong stimulation of photosynthesis throughout the growing season, especially for B2 leaves but not by extending or shortening the growing season in autumn.