Azimuthal and radial variations in sap flux density and effects on stand-scale transpiration estimates in a Japanese cedar forest

Understanding radial and azimuthal variation, and tree-to-tree variation, in sap flux density (F_d) as sources of uncertainty is important for estimating transpiration using sap flow techniques. In a Japanese cedar (Cryptomeria japonica D. Don.) forest, F_d was measured at several depths and aspects for 18 trees, using heat dissipation (Granier-type) sensors. We observed considerable azimuthal variation in F_d. The coefficient of variation (CV) calculated from F_d at a depth of 0-20 mm (F _d1) and F_d at a depth of 20-40 mm (F_d2) ranged from 6.7 to 37.6% (mean = 28.3%) and from 19.6 to 62.5% (mean = 34.6%) for the azimuthal directions. F_d at the north aspect averaged for nine trees, for which azimuthal measurements were made, was obviously smaller than F _d at the other three aspects (i.e., west, south and east) averaged for the nine trees. F_d1 averaged for the nine trees was significantly larger than F_d2 averaged for the nine trees. The error for stand-scale transpiration (E) estimates caused by ignoring the azimuthal variation was larger than that caused by ignoring the radial variation. The error caused by ignoring tree-to-tree variation was larger than that caused by ignoring both radial and azimuthal variations. Thus, tree-to-tree variation in F_d would be more important than both radial and azimuthal variations in F_d for E estimation. However, F_d for each tree should not be measured at a consistent aspect but should be measured at various aspects to make accurate E estimates and to avoid a risk of error caused by the relationship of F_d to aspect.