Abstract

Several studies have suggested that CO₂ transport in the transpiration stream can considerably bias estimates of root and stem respiration in ring-porous and diffuse-porous tree species. Whether this also happens in species with tracheid xylem anatomy and lower sap flow rates, such as conifers, is currently unclear. We infused ¹³C-labelled solution into the xylem near the base of two 90-year-old Pinus sylvestris L. trees. A custom-built gas exchange system and an online isotopic analyser were used to sample the CO₂ efflux and its isotopic composition continuously from four positions along the bole and one upper canopy shoot in each tree. Phloem and needle tissue ¹³C enrichment was also evaluated at these positions. Most of the ¹³C label was lost by diffusion within a few metres of the infusion point indicating rapid CO₂ loss during vertical xylem transport. No ¹³C enrichment was detected in the upper bole needle tissues. Furthermore, mass balance calculations showed that c. 97% of the locally respired CO₂ diffused radially to the atmosphere. Our results support the notion that xylem CO₂ transport is of limited magnitude in conifers. This implies that the concerns that stem transport of CO₂ derived from root respiration biases chamber-based estimates of forest carbon cycling may be unwarranted for mature conifer stands.

中文翻译：

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在成熟的北方樟子松树的茎中，垂直CO2的运输受到限制。

摘要

几项研究表明，蒸腾流中的CO ₂转运可能会严重估计环孔和散孔树种的根和茎呼吸。目前尚不清楚是否在气管木质部解剖结构和较低的树液流量的物种（例如针叶树）中发生这种情况。我们将¹³ C标记的溶液注入到两棵90岁的樟子松树基部附近的木质部中。使用定制的气体交换系统和在线同位素分析仪，从沿树干的四个位置和每棵树中的一个上部树冠芽连续采样CO ₂外排及其同位素组成。韧皮部和针头组织¹³在这些位置也评估了C富集。¹³ C标签的大部分由于在输液点几米内的扩散而损失掉，表明在垂直木质部运输过程中CO ₂迅速损失。没有¹³上部树干针组织中检测到Ç富集。此外，质量平衡计算表明c。97％的局部呼吸的CO ₂径向扩散到大气中。我们的结果支持了针叶树中木质部CO _2的运输量有限的观点。这意味着对于成熟的针叶林林分而言，根系呼吸作用导致的CO _2的茎运输偏向基于室的森林碳循环估计的担忧可能不再成立。