Mildner Manuel, Bader Martin K-F, Leuzinger Sebastian, Siegwolf Rolf T W, Körner Christian
Institute of Botany, University of Basel, Schönbeinstrasse 6, 4056, Basel, Switzerland,
Oecologia. 2014 Jul;175(3):747-62. doi: 10.1007/s00442-014-2935-5. Epub 2014 Apr 3.
There is evidence of continued stimulation of foliage photosynthesis in trees exposed to elevated atmospheric CO2 concentrations; however, this is mostly without a proportional growth response. Consequently, we lack information on the fate of this extra carbon (C) acquired. By a steady application of a (13)CO2 label in a free air CO2 enrichment (FACE) experiment, we traced the fate of C in 37-m-tall, ca. 110-year-old Picea abies trees in a natural forest in Switzerland. Hence, we are not reporting tree responses to elevated CO2 (which would require equally (13)C labeled controls), but are providing insights into assimilate processing in such trees. Sunlit needles and branchlets grow almost exclusively from current assimilates, whereas shaded parts of the crowns also rely on stored C. Only 2.5 years after FACE initiation, tree rings contained 100% new C. Stem-respiratory CO2 averaged 50% of new C over the entire FACE period. Fine roots and mycorrhizal fungi contained 49-56 and 26-43% new C, respectively, after 2.5 years. The isotopic signals in soil CO2 arrived 12 days after the onset of FACE, yet it contained only ca. 15% new C thereafter. We conclude that new C first feeds into fast turnover C pools in the canopy and becomes increasingly mixed with older C sources as one moves away (downward) from the crown. We speculate that enhanced C turnover (its metabolic cost) along the phloem path, as evidenced by basipetal isotope signal depletion, explains part of the 'missing carbon' in trees that assimilated more C under elevated CO2.
有证据表明，暴露于大气二氧化碳浓度升高环境中的树木叶片光合作用会持续受到刺激；然而，这种刺激大多并未带来成比例的生长响应。因此，我们缺乏关于这些额外获取的碳（C）去向的信息。通过在自由空气二氧化碳富集（FACE）实验中持续施加（13）CO2标记，我们追踪了瑞士一片天然森林中37米高、约110年树龄的欧洲云杉树木中碳的去向。因此，我们并非在报告树木对二氧化碳浓度升高的响应（这需要同样用（13）C标记的对照），而是在提供关于此类树木同化物处理的见解。受阳光照射的针叶和小枝几乎完全由当前的同化物生长而来，而树冠的荫蔽部分也依赖储存的碳。FACE实验开始仅2.5年后，树木年轮就含有100%的新碳。在整个FACE实验期间，树干呼吸产生的二氧化碳平均占新碳的50%。2.5年后，细根和菌根真菌分别含有49 - 56%和26 - 43%的新碳。土壤二氧化碳中的同位素信号在FACE实验开始12天后出现，但此后其仅含有约15%的新碳。我们得出结论，新碳首先进入树冠中快速周转的碳库，并且随着从树冠向下移动，它与较老的碳源混合得越来越多。我们推测，韧皮部路径上碳周转的增强（其代谢成本），如向基同位素信号消耗所证明的，解释了在二氧化碳浓度升高条件下吸收更多碳的树木中“缺失碳”的部分原因。
