Can we assume insignificant temporal changes in spatial variations of sap flux for year-round individual tree transpiration estimates? A case study on Cryptomeria japonica in central Taiwan

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 ( F d ) affect the accuracy of year-round individual tree water use ( Q ) estimates. We aimed to determine seasonal changes in radial and azimuthal variations in F d and to evaluate their potential impacts on year-round Q estimates for humid subtropical forest trees. We measured F d using 38 probes in eight Cryptomeria japonica trees in Taiwan from July 2010 to May 2011. During the study period, the ratio of inner F d to outermost F d ( R r ) ranged between 0.29 and 0.68. The ratio of mean outermost F d to that at one direction ( R a ) 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 ( Q m ) with those derived from measurements at a certain position with constant correction factors for radial ( Q r ) or azimuthal ( Q a ) patterns of F d determined from the summer measurements. This simple exercise revealed consistent relationships between Q m and Q r year-round (<10% error), suggesting that seasonal changes in the radial profiles of F d had little effect on Q estimates. Although using a constant correction factor for azimuthal variations led to 20–40% errors in Q a of some individuals with large azimuthal variations (i.e., CV > 0.49), five out of eight individuals had less than 10% errors in their Q a 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 F d .