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.

Institute of Botany Faculty of Biology Jagiellonian University Kopernika 27 31 501 Kraków Poland

Swiss Federal Research Institute for Forest Snow and Landscape Research Zürcherstrasse 111 CH 8903 Birmensdorf Switzerland

Swiss Federal Research Institute for Forest Snow and Landscape Research Zürcherstrasse 111 CH 8903 Birmensdorf Switzerland Department of Ecology W Szafer Institute of Botany Polish Academy of Sciences ul Lubicz 46 31 512 Kraków Poland

Swiss Federal Research Institute for Forest Snow and Landscape Research Zürcherstrasse 111 CH 8903 Birmensdorf Switzerland Department of Environmental Systems Science Institute of Agricultural Sciences ETH Zurich 8092 Zurich Switzerland Faculty of Forestry and Wood Sciences Czech University of Life Sciences Kamýcka Cesta 1176 CZ 165 21 Praha 6 Suchdol Czech Republic

Swiss Federal Research Institute for Forest Snow and Landscape Research Zürcherstrasse 111 CH 8903 Birmensdorf Switzerland Laboratory of Plant Ecology Faculty of Bioscience Engineering Ghent University Coupure links 653 B 9000 Ghent Belgium

Technical University of Denmark Department of Environmental Engineering Air Land and Water Resources Section Bygningstorvet 115 2800 Kgs Lyngby Denmark

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