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

Abstract

Understanding radial and azimuthal variation, and tree-to-tree variation, in sap flux density (Fd) as sources of uncertainty is important for estimating transpiration using sap flow techniques. In a Japanese cedar (Cryptomeria japonica D. Don.) forest, Fd was measured at several depths and aspects for 18 trees, using heat dissipation (Granier-type) sensors. We observed considerable azimuthal variation in Fd. The coefficient of variation (CV) calculated from Fd at a depth of 0-20 mm (Fd1) and Fd at a depth of 20-40 mm (Fd2) ranged from 6.7 to 37.6% (mean = 28.3%) and from 19.6 to 62.5% (mean = 34.6%) for the -azimuthal directions. Fd at the north aspect averaged for nine trees, for which azimuthal measurements were made, was -obviously smaller than Fd at the other three aspects (i.e., west, south and east) averaged for the nine trees. Fd1 averaged for the nine trees was significantly larger than Fd2 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 Fd would be more important than both radial and azimuthal variations in Fd for E estimation. However, Fd 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 Fd to aspect.

Stand Transpiration Estimates from Recalibrated Parameters for the Granier Equation in a Chinese Fir (Cunninghamia lanceolata) Plantation in Southern China

“…Several studies have reported the existence of significant relationships between tree characteristics (i.e., tree height or DBH) and the water use of trees [34,39]. These relationships could be explained by energy limitations in our study plot.…”

“…All stem segments were processed according to the procedure followed by Shinohara et al [34]. Constant heads of water of 10,20,30,40,60,80, and 100 cm were maintained on each cut stem segment for at least half an hour during the experiments.…”

Observations of stem water storage in trees of opposing hydraulic strategies

“…In future studies, the use of multiple probe lengths may be most useful to fully capture storage at all radial depths. The use of multiple probe lengths is common in sap flux studies to capture differing flow velocities with depth (e.g., Phillips et al 1996, Schäfer et al 2000, Shinohara et al 2013.…”

Species‐specific transpiration responses to intermediate disturbance in a northern hardwood forest

“…On this basis, we expected an overestimation of E C of roughly 11% in the control plot and 3.7% in the treatment plot. Due to the dependence of radial patterns in sap flow on species, tree age, and site history and, therefore, the large variability among reduction factors, we chose to disclose this limitation rather than to assume the applicability of a reduction factor from existing literature that was not specific to our plots [James et al, 2002;Nadezhdina et al, 2002;Phillips et al, 1996;Renninger et al, 2013;Schäfer et al, 2000;Shinohara et al, 2013]. Despite these errors, the roughly 15% difference in transpiration between the control and treatment plots remains significant.…”