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

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 J _max in 1-year-old needles (P < 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].