Key message: Although spatial variations in sap flux changed seasonally in cedar trees, neglecting these seasonal changes generally resulted in less than 10% errors in year-round tree transpiration estimates. Abstract: Spatial and temporal variations in sap flux (Fd) affect the accuracy of year-round individual tree water use (Q) estimates. We aimed to determine seasonal changes in radial and azimuthal variations in Fd and to evaluate their potential impacts on year-round Q estimates for humid subtropical forest trees. We measured Fd using 38 probes in eight Cryptomeria japonica trees in Taiwan from July 2010 to May 2011. During the study period, the ratio of inner Fd to outermost Fd (Rr) ranged between 0.29 and 0.68. The ratio of mean outermost Fd to that at one direction (Ra) ranged between 0.36 and 1.89. The seasonal patterns were inconsistent among individuals. We compared year-round Q estimates derived from multi-sensor measurements (Qm) with those derived from measurements at a certain position with constant correction factors for radial (Qr) or azimuthal (Qa) patterns of Fd determined from the summer measurements. This simple exercise revealed consistent relationships between Qm and Qr year-round (<10% error), suggesting that seasonal changes in the radial profiles of Fd had little effect on Q estimates. Although using a constant correction factor for azimuthal variations led to 20–40% errors in Qa of some individuals with large azimuthal variations (i.e., CV > 0.49), five out of eight individuals had less than 10% errors in their Qa estimates. These results suggested that constant correction factors can be used for radial variations in year-round Q estimates, but are not applicable for azimuthal variations in individuals showing large azimuthal variations of Fd.
All Science Journal Classification (ASJC) codes
- Plant Science