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

被引:63
作者
Crous, KY [1 ]
Ellsworth, DS [1 ]
机构
[1] Univ Michigan, Sch Nat Resources & Environm, Ann Arbor, MI 48109 USA
关键词
canopy CO2 exchange; conifer forest; down-regulation; Forest Free-Air CO2 Enrichment; leaf age; leaf carboxylation rate; leaf nitrogen; net photosynthesis;
D O I
10.1093/treephys/24.9.961
中图分类号
S7 [林业];
学科分类号
0829 ; 0907 ;
摘要
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 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 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].
引用
收藏
页码:961 / 970
页数:10
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