Maximum heat ratio: bi-directional method for fast and slow sap flow measurements

Background: As sap flow research expands, new challenges such as fast sap flows or flows co-occurring with freeze/thaw cycles appear, which are not easily addressed with existing methods. In order to address these new challenges, sap flow methods capable of measuring bidirectional, high and slow sap flux densities (F_d, cm³ cm⁻² h⁻¹), thermal properties and stem water content with minimum sensitivity to stem temperature are required. Purpose: In this study we assessed the performance of a new low-power ratio-based algorithm, the maximum heat ratio (MHR) method, and compare it with the widely known heat ratio (HR) method using a cut-tree study to test it under high flows using Eucalyptus grandis trees, and a freeze/thaw experiment using Acer saccharum trunks to test its response to fast changing stem temperatures that result in freeze/thaw cycles. Results: Our results indicate that MHR and HR had a strong (R² = 0.90) linear relationship within a F_d range of 0–45 cm³ cm⁻² h⁻¹. Using the MHR algorithm, we were able to estimate wood thermal properties and water content, while extending the measuring range of HR to approximately 0–130 (cm³ cm⁻² h⁻¹). In our freeze/thaw experiment, the main discrepancy between MHR and HR was observed during freezing, where HR had consistently lower F_d (up to 10 cm³ cm⁻² h⁻¹), with respect to MHR. However, both algorithms identified similar zero flows. Conclusion: Consequently, MHR can be an easy-to-implement alternative algorithm/method capable of handling extreme climatic conditions, which can also run simultaneously with HR.