Data from: Axial xylem architecture of Larix decidua exposed to CO2 enrichment and soil warming at the treeline

1. Trees continuously adjust their axial xylem structure to meet changing needs imposed by ontogenetic and environmental changes. These axial structure-function responses need to be coordinated among competing biophysical constraints to avoid failure of the xylem system. Here, we investigated if ontogeny or experimental manipulation of CO2 and soil temperature influence these structure-function responses. 2. We performed detailed xylem cell anatomical quantification along the axis of 40-year-old Larix decidua trees planted at the Swiss treeline and exposed to a combination of elevated CO2 (+200 ppm) and soil warming (+4 °C) between 2001 and 2012. We assessed how mean hydraulic tracheid diameter (Dh), the cell wall reinforcement ((t/b)2), tracheid wall thickness (CWT) and the percent area of ray parenchyma (PERPAR) – proxies for hydraulic efficiency, hydraulic safety, biomechanical support and metabolic xylem functions, respectively – co-vary along the tree axis. 3. Dh increased from the stem apex to base, strictly following a power function (R2=0.81), independent from ontogeny and experimental treatments. In contrast, axial trends of (t/b)2 and CWT were either influenced by treatment and/or ontogeny, or showed no axial trend (PERPAR). Additionally, we found that a larger Dh only at the stem apex promoted primary and secondary growth. 4. Our approach of analyzing xylem anatomical traits along the tree axis and across tree-rings provides novel insights into xylem functional architecture and allows reconstructing xylem function over time. We conclude that the maintenance of hydraulic efficiency during ontogeny is very robust, as the conduit diameter undergoes a strong apical control, and plays a fundamental role for assimilation and tree growth. Instead, the other functional traits more plastically vary with ontogeny and environmental changes.

1. 树木会持续调整其轴向木质部结构，以适应个体发育（ontogeny）与环境变化带来的动态需求。这类轴向结构-功能响应需要在相互制衡的生物物理约束间进行协调，以避免木质部系统出现功能失效。本研究旨在探究个体发育，或是通过实验操控CO₂浓度与土壤温度，是否会对这类结构-功能响应产生影响。
2. 我们以2001年至2012年间，种植于瑞士林木线（treeline）处、接受CO₂浓度升高（+200 ppm）与土壤增温（+4℃）复合处理的40年生欧洲落叶松（Larix decidua）为研究对象，沿其树干轴向开展了细致的木质部细胞解剖学定量分析。我们分别测定了平均输水管胞直径（Dh）、细胞壁加固系数((t/b)²)、管胞壁厚（CWT）以及射线薄壁细胞面积占比（PERPAR）——这四项指标依次分别作为输水效率、输水安全性、生物力学支撑以及木质部代谢功能的替代表征——并分析它们沿树干轴向的协同变化规律。
3. 平均输水管胞直径（Dh）从茎尖到茎基逐渐增大，严格遵循幂函数分布（决定系数R²=0.81），且不受个体发育与实验处理的影响。与之相对，细胞壁加固系数((t/b)²)与管胞壁厚（CWT）的轴向变化趋势要么受到实验处理和/或个体发育的调控，要么不存在显著轴向变化规律（如射线薄壁细胞面积占比PERPAR）。此外我们还发现，仅在茎尖增大的管胞直径可促进树木的初生与次生生长。
4. 我们沿树干轴向且跨年轮分析木质部解剖性状的研究方法，为木质部功能架构提供了全新的认知视角，同时可实现木质部功能的时间序列重建。本研究结论表明，在个体发育过程中输水效率的维持具有极高的稳定性——这是由于管胞直径受到强烈的顶端调控，且该过程对树木的光合同化与生长发育发挥着核心作用。与之相反，其余功能性状则会随个体发育与环境变化呈现出更强的可塑性变异。