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Few studies have examined the effects of elevated CO 2  concentration ([CO 2 ]) on the physiology of intact forest canopies, despite the need to understand how leaf-level responses can be aggregated to assess effects on whole-canopy functioning. We examined the long-term effects of elevated [CO 2 ] (ambient + 200 ppm CO 2 ) on two age classes of needles in the upper and lower canopy of  Pinus taeda  L. during the second through sixth year of exposure to elevated [CO 2 ] in free-air (free-air CO 2  enrichment (FACE)) in North Carolina, USA. Strong photosynthetic enhancement in response to elevated [CO 2 ] (e.g., +60% across age classes and canopy locations) was observed across the years. This stimulation was 33% greater for current-year needles than for 1-year-old needles in the fifth and sixth years of treatment. Although photosynthetic stimulation in response to elevated [CO 2 ] was maintained through the sixth year of exposure, we found evidence of concurrent down-regulation of Rubisco and electron transport capacity in the upper-canopy sunlit leaves. The lower canopy showed no evidence of down-regulation. The upper canopy down-regulated carboxylation capacity ( V  cmax ) and electron transport capacity ( J  max ) by about 17–20% in 1-year-old needles; however, this response was significant across sampling years only for Jmax in 1-year-old needles ( P  &lt; 0.02). A reduction in leaf photosynthetic capacity in aging conifer needles at the canopy top could have important consequences for canopy carbon balance and global carbon sinks because 1-year-old sunlit needles contribute a major proportion of the annual carbon balance of these conifers. Our finding of a significant interaction between canopy position and CO 2  treatment on the biochemical capacity for CO 2  assimilation suggests that it is important to take canopy position and needle aging into account because morphologically and physiologically distinct leaves could respond differently to elevated [CO 2 ].

Canopy position affects photosynthetic adjustments to long-term elevated CO 2 concentration (FACE) in aging needles in a mature Pinus taeda forest