Quantification of uncertainties introduced by data-processing procedures of sap flow measurements using the cut-tree method on a large mature tree

Sap flow sensors are crucial instruments to understand whole-tree water use. The lack of direct calibration of the available methods on large trees and the application of several data-processing procedures may jeopardize our understanding of water uptake dynamics by increasing the uncertainties around sensor-based estimates. We directly compared the heat ratio method (HRM) sap flow measurements to water uptake measured gravimetrically using the cut-tree method on a large mature aspen tree to quantify those uncertainties for ten consecutive days. The influence of the azimuthal position of the sensors and the application of different data-processing procedures on the accuracy of the sap flow sensors' estimates was assessed using different metrics. Overall, the sap flow measurements showed high temporal precision with the gravimetric data. Azimuthal and radial variability of measured sap flux density showed the most substantial effect on the accuracy of the sensors' estimates of whole tree sap flow. The zero-flow corrections applied altered the accuracy and linearity of the sensors' measurements at the hourly scale, while the sapwood area method used had a lesser impact. Across the ensemble of available data-processing procedures, the cumulative whole-tree water uptake estimates for five consecutive days from the sensor diverged from the gravimetric measurements by less than 1% to more than 50% depending on the sensors azimuthal position and data corrections applied. This study illustrates some of the uncertainties associated with the methodological approaches chosen when using sap flow sensors to estimate water uptake in tall and large diameter trees.