Field evaluation of an in-situ probe spacing and deflection correction method for sap flow measurement

• Ren et al. (2022) ’s real-time in situ correction method significantly enhances the accuracy of sap flow rate estimation;. • Probe deflection reliably diagnosed by >1 cm h⁻¹ V h fluctuation in 60–100 s window;. • Formulated species-specific operational guidelines for long-term monitoring. The accuracy of the widely used heat pulse method for sap flow estimation is highly sensitive to errors in probe spacing, which can be caused by imperfect insertion or subsequent tree growth. To address this, Ren et al. (2022) developed a promising real-time, in-situ calibration technique involving nonlinear curve fitting during low flow conditions. However, the practical efficacy of this method requires further validation. Here, we rigorously assess its accuracy and field applicability. The results show that fluctuations in ln(Δ T d /Δ T u ) or V h values during the 60–100 s interval can reliably indicate probe deflection: when V h fluctuations exceed 1 cm h⁻¹, probe deflection is likely. Numerical simulations demonstrate that Ren et al. (2022)’s method effectively corrects various types of probe deflection. In field experiments, the method not only reduces V h fluctuations in the 60–100 s window but also decreases the deviation between pre- and post-correction values to 3.9 cm h⁻¹ for apple trees and 5.6 cm h⁻¹ for poplar trees. For long-term experiments, we recommend correcting probe spacing every three months for fast-growing trees and every six months for slow-growing trees. Regardless of the interval, this method provides real-time calibration. Thus, Ren et al. (2022)’s real-time in situ correction method significantly enhances the accuracy of sap flow rate estimation and offers robust technical support for research on tree water use.