During the growing season, trees allocate photoassimilates to increase their aboveground woody biomass in the stem (ABIstem). This 'carbon allocation' to structural growth is a dynamic process influenced by internal and external (e.g., climatic) drivers. While radial variability in wood formation and its resulting structure have been intensively studied, their variability along tree stems and subsequent impacts on ABIstem remain poorly understood. We collected wood cores from mature trees within a fixed plot in a well-studied temperate Fagus sylvatica L. forest. For a subset of trees, we performed regular interval sampling along the stem to elucidate axial variability in ring width (RW) and wood density (ρ), and the resulting effects on tree- and plot-level ABIstem. Moreover, we measured wood anatomical traits to understand the anatomical basis of ρ and the coupling between changes in RW and ρ during drought. We found no significant axial variability in ρ because an increase in the vessel-to-fiber ratio with smaller RW compensated for vessel tapering towards the apex. By contrast, temporal variability in RW varied significantly along the stem axis, depending on the growing conditions. Drought caused a more severe growth decrease, and wetter summers caused a disproportionate growth increase at the stem base compared with the top. Discarding this axial variability resulted in a significant overestimation of tree-level ABIstem in wetter and cooler summers, but this bias was reduced to ~2% when scaling ABIstem to the plot level. These results suggest that F. sylvatica prioritizes structural carbon sinks close to the canopy when conditions are unfavorable. The different axial variability in RW and ρ thereby indicates some independence of the processes that drive volume growth and wood structure along the stem. This refines our knowledge of carbon allocation dynamics in temperate diffuse-porous species and contributes to reducing uncertainties in determining forest carbon fixation.

中文翻译：

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木材功能性状的轴向变化对欧洲山毛榉（Fagus sylvatica）的茎生物量增加的净效应有限。

在生长季节，树木会分配光同化物以增加茎中的地上木质生物量（ABIstem）。这种对结构性增长的“碳分配”是一个受内部和外部（例如气候）驱动因素影响的动态过程。尽管已经深入研究了木材形成的径向变异性及其形成的结构，但对沿树茎的变异性及其对ABIstem的影响仍然知之甚少。我们在经过精心研究的温带温带水青冈森林中的固定地块中，从成熟树木中收集木芯。对于树木的子集，我们沿茎进行了定期的间隔采样，以阐明环宽度（RW）和木材密度（ρ）的轴向变化，以及对树木和样地级ABI系统的影响。此外，我们测量了木材的解剖特征，以了解ρ的解剖基础以及干旱期间RW和ρ变化之间的耦合。我们发现ρ没有明显的轴向变化，因为随着较小的RW的容器与纤维之比的增加补偿了容器向顶点逐渐变细。相反，RW的时间变异性沿茎轴的变化很大，这取决于生长条件。与干旱相比，干旱导致更严重的生长下降，而夏季潮湿导致茎基部的生长增加不成比例。抛弃这种轴向变化会导致在潮湿和凉爽的夏季显着高估树木水平的ABIstem，但是当将ABIstem缩放到地块水平时，这种偏差减少到〜2％。这些结果表明F。在不利条件下，西尔维蒂察优先考虑靠近树冠的结构性碳汇。RW和ρ的不同轴向变化从而表明了沿茎部推动体积增长和木材结构的过程的某些独立性。这丰富了我们对温带弥散多孔物种碳分配动态的了解，并有助于减少确定森林碳固定的不确定性。