1. Leaf venation is a key trait determining leaf water relations, phloem transport and mechanical stability. However, since the contribution of leaf veins to mechanical properties has not been properly understood, our understanding of how leaf vein architecture adjusts these functions within leaves and across species as well as its mechanistic basis is also limited. Here, we tested the hypotheses that leaf water-use and mechanical properties would be more correlated with lower-and higher-order vein density (VLA), respectively. 2. We studied eight Fagaceae species growing in temperate forests in Japan and differing in secondary vein patterns, leaf habits, size and natural habitat. We quantified two leaf water-use traits [lamina hydraulic conductance (K lamina ) and carbon stable isotope ratio (d 13 C)], two mechanical properties [lamina tensile strength and tensile modulus of elasticity (E)] and nine traits relating to both vein architecture and leaf structure. 3. Across species, primary and secondary vein density (VLA 1Á2 ) correlated positively to lamina strength and leaf structural traits (leaf area and leaf mass per area) but not to water use. In contrast, minor vein density (VLA min ) correlated to K lamina positively but not to mechanical properties and leaf structural traits. Water-use traits and mechanical properties were independent on both area and mass basis. 4. Our results indicate that there is labour sharing in leaf functions among higher-and lowerorder veins, that VLA 1Á2 influences leaf structural and mechanical properties and that VLA min affects leaf water use and, thus, potentially the gas exchange capacity. These findings suggest that vein hierarchy is associated with functional differentiation among vein orders to support leaf functions. This system possesses different vein orders that are quantitatively and qualitatively different within leaves to carry out multiple functions. Therefore, the evolutionary or accumulative shift in vein hierarchy might explain the variation in leaf structure and functions.

Parenchyma is an important component of the secondary xylem. It has multiple functions and its fraction is known to vary substantially across angiosperm species. However, the physiological significance of this variation is not yet fully understood. Here, we examined how different types of parenchyma (ray and axial parenchyma, and axial parenchyma in direct contact with vessels [APV]) are coordinated with three essential xylem functions: water conduction, storage of non-structural carbohydrate (NSC), and mechanical support. Using branch sapwood of 15 co-occurring drought-adapted woody species from the subtropical Bonin Islands, Japan, we quantified ten xylem anatomical traits and examined their linkages to hydraulic properties, storage of soluble sugars and starch, and sapwood density. The fractions of APV and axial parenchyma in the xylem transverse sections were positively correlated with the percentage loss of conductivity in the native condition, whereas that of ray parenchyma was negatively correlated with the maximum conductivity across species. Axial and ray parenchyma fractions were positively associated with concentrations of starch and NSC. The fraction of parenchyma was independent of sapwood density, regardless of parenchyma type. We also identified a negative relationship between hydraulic conductivity and NSC storage and sapwood density, mirroring the negative relationship between the fractions of parenchyma and vessels. These results suggest that parenchyma fraction underlies species variation in xylem hydraulic and carbon use strategies, wherein xylem with a high fraction of axial parenchyma may adopt an embolism repair strategy through an increased starch storage with low cavitation resistance.

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