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Canopy position affects photosynthetic adjustments to long-term elevated CO₂ concentration (FACE) in aging needles in a mature Pinus taeda forest

Kristine Y. Crous (1) and David S. Ellsworth (1, 2)

1. School of Natural Resources and Environment, University of Michigan, 430 E. University Avenue, Ann Arbor, MI 48109-1115, USA / 2. Corresponding author (ellswor@umich.edu) / Received October 29, 2003; accepted January 26, 2004; published online July 1, 2004

Summary

Few studies have examined the effects of elevated CO₂ concentration ([CO₂]) 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 [CO₂] (ambient + 200 ppm CO₂) 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 [CO₂] in free-air (free-air CO₂ enrichment (FACE)) in North Carolina, USA. Strong photosynthetic enhancement in response to elevated [CO₂] (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 [CO₂] 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 < 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 CO₂ treatment on the biochemical capacity for CO₂ 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 [CO₂].

Keywords: canopy CO₂ exchange, conifer forest, down-regulation, Forest Free-Air CO₂ Enrichment, leaf age, leaf carboxylation rate, leaf nitrogen, net photosynthesis.

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ISSN 0829-318X Copyright © 2002–2008 Heron Publishing