Xylem vessel anatomy and hydraulic function scale in concert along the tip-to-base axis of an angiosperm tree

Vessel scaling from tip to base in angiosperms has largely been studied based on vessel diameter. Here, we test if vessel anatomy and transport efficiency in a Fagus sylvatica L. sapling show axial scaling by maintaining a largely proportional ratio of lumen to end-wall resistivity to sap flow with tree height. Vessel diameter (D) of more than 50 000 vessels was measured based on wood sections, while mean vessel length (L_V ) was measured semi-automatically with a Pneumatron for 58 stem segments. Based on tip-to-base variation in D and L_V , we estimated vessel lumen conductivity (K_H ) at the individual vessel level. We also estimated end-wall conductivity (K_W ) based on Darcy's law, integrating pit membrane thickness (T _PM) with scaling of D and total inter-vessel pit membrane area (A_P ) across the sapling. Axial variation in K_W was evaluated against end-wall pressure difference ( $\Delta P$ ). In addition to a tip-to-base increase in D, we found an increase in L_V and A_P , illustrating basipetal vessel lengthening. These patterns were associated with proportional changes in K_W and K_H , which followed a 1:1 relationship with distance to the tip, each contributing to ∼ 50% of the whole-tree conductivity/resistivity. Our findings suggest that vessel dimensions and hydraulic functionality show axial scaling in angiosperm trees, suggesting that anatomy corresponds to the adjustment of hydraulic functionality with plant height. Proportional adjustment of K_W and K_H highlights the key role of vessel dimensions and inter-vessel pits in regulating transport efficiency and safety, potentially maintaining constant resistance per unit leaf area with height growth.