Data from: Growth and carbon relations of mature Picea abies trees under five years of free air CO2 enrichment

• Are mature forests carbon limited? To explore this question, we exposed ca. 110 year-old, 40 m tall Picea abies trees to a 550 ppm CO2 concentration in a mixed lowland forest in NW Switzerland. The site receives substantial soluble nitrogen (N) via atmospheric deposition, and thus, trees are unlikely N-limited. We used a construction crane to operate the free air CO2 release system and for canopy access. Here we summarize the major results for growth and carbon (C) fluxes.
• Tissue 13C signals confirmed the effectiveness of the CO2 enrichment system and permitted tracing the continuous flow of new C in trees. Tree responses were individually standardized by pretreatment signals. Over the 5 experimental years, needles retained their photosynthetic capacity and absorbed up to 37% more CO2 under elevated (E) compared to ambient (A) conditions. However we did not detect an effect on stem radial growth, branch apical growth, and needle litter production. Neither stem nor soil CO2 efflux was stimulated under elevated CO2. The rate at which fine roots filled soil ingrowth cores did not significantly differ between A and E trees.
• Since trees showed no stomatal responses to elevated CO2, sap flow remained unresponsive, both in the long run as well as during short-term CO2 on-off experiments. As a consequence, soil moisture remained unaffected. We trapped significantly more nitrate in the root sphere of E-trees suggesting a CO2-stimulated breakdown of soil organic matter, presumably induced by extra carbohydrate exudation (‘priming’).
• Synthesis. The lack of a single enhanced C sink to match the increased C uptake meant a missing C sink. Increased C transport to belowground sinks was indicated by C transfer to ectomycorrhiza and on to neighboring trees and by increased C export to soil. We conclude that these tall Picea abies trees are not C limited at current CO2 concentrations and further atmospheric CO2 enrichment will have at most subtle effects on growth, despite enhanced N availability.

• 成熟森林是否受碳限制？为探究这一问题，我们在瑞士西北部的一处混合低地森林中，对树龄约110年、树高40米的欧洲云杉（Picea abies）施加550 ppm的CO₂浓度处理。该样地通过大气沉降获得大量可溶性氮（N），因此树木不太可能受氮限制。我们借助施工起重机操作自由空气CO₂释放系统（free air CO₂ release system），并用于林冠作业。本研究总结了生长与碳（C）通量（carbon fluxes）的主要结果。
• 组织13C信号验证了CO₂富集系统的有效性，并可追踪树木中新碳的连续流动过程。通过预处理信号对树木的响应进行个体标准化处理。在5年的实验周期内，与环境（A）组相比，升高（E）CO₂浓度下的针叶仍维持光合能力，CO₂吸收量最多提升37%。但我们未检测到其对茎径向生长、枝条顶端生长以及针叶凋落物产量的影响。升高CO₂浓度并未促进茎部或土壤的CO₂排放。细根填充土壤生长芯的速率在环境组与升高组树木间无显著差异。
• 由于树木对升高的CO₂浓度无气孔响应，无论长期还是短期CO₂通断实验中，树液流量均未发生变化。因此，土壤湿度也未受影响。我们在升高CO₂组树木的根际区域捕获到更多硝酸盐，这表明CO₂富集刺激了土壤有机质的分解，推测该过程由额外的碳水化合物渗出引发（即"激发效应（priming）"）。
• 综合分析。由于缺乏可匹配碳吸收增量的有效碳汇，导致出现碳汇缺失现象。碳向外生菌根（ectomycorrhiza）及邻近树木的转运，以及碳向土壤的输出增加，均表明碳向地下汇的运输有所增强。我们得出结论：在当前CO₂浓度下，这些高大的欧洲云杉并未受碳限制，且尽管氮有效性有所提升，大气CO₂进一步富集对其生长的影响至多仅为细微变化。