Spatial variations in xylem sap flux density in evergreen oak trees with radial-porous wood: Comparisons with anatomical observations

To estimate whole-tree water use when employing sap flow measurements, integration of the sap flux density (F d) over the sapwood area is needed. Accordingly, it is necessary to obtain information on the characteristics of stem water transportation such as spatial variations in F d and the active xylem area in the stem cross-section. Although evergreen oak trees with radial-porous wood represent a major component of secondary forests in western Japan, detailed information on their stem water transportation characteristics remains unclear. In the present study, we used the heat dissipation method (Granier method) to conduct measurements of azimuthal and radial variations in the F d of Quercus glauca Thunb. ex Murray, a representative evergreen broad-leaved tree in western Japan. Further, by analyzing the anatomy of the xylem structure, we examined why F d varies spatially in the stem cross-section. By using a dye solution injected into a radial hole bored into the tree trunk, we confirmed that the entire stem is hydroactive. We also compared the spatial variations in F d and water conductivity per xylem area (K s) which were estimated by using the observed vessel diameters and their density over the stem cross-section and Hagen–Poiseuille’s law. Azimuthal and radial variations in F d reached about 60 and 50% of the maximum values, respectively, and could be explained by spatial variation in K s. As a result, we obtained statistical parameters describing the spatial variation in F d in Q. glauca and determined that whole-tree water use estimated from measurements in one direction had at most ±20% potential errors for studied trees.