A new consistent sap flow baseline-correction approach for the stem heat balance method using nocturnal water vapour pressure deficits and its application in the measurements of urban climbing plant transpiration

- Qfic and its detection limits for different flow rates and Tair.
- A new calibration method for sap flow data using minimum nocturnal VPDs.
- Testing different calibration approaches by simultaneous weight measurements.

The stem heat balance (SHB) method is a widely used sap flow technique to determine the transpiration and the water demands of herbaceous and woody plants, especially those with small diameters (e.g. climbers). The accuracy of the sap flow derived by this method (QS) depends on correction of the total measured heat input (Qt) by subtracting unintended heat losses; these heat losses are referred to as “fictitious flow” (Qfic) (QS = Qt − Qfic).We developed a physically consistent baseline-correction approach using minimum nocturnal water vapour pressure deficits (VPD). This VPD approach was compared to the so-called “night value subtraction” (NVS) approach and direct gravimetric determination for potted climbing plants and an outdoor climbing plant stand. In addition, performance tests were also conducted on artificial model stems and cut plant stems.In the tests with the outdoor climbing plant stand, sap flow corrected by the NVS approach underestimated daily transpiration by up to 33% compared to direct gravimetric determination. In contrast, the newly developed VPD approach underestimated or overestimated transpiration by only 5%-10%. The VPD approach makes use of the direct dependence of sap flow on VPD during zero-radiation conditions (night). This means, Qfic is the constant of the linear regression of the VPD and the lowest recorded Qt at night. Therefore, the correction is based on all recorded sap flow data from the measurement period itself, which in turn accounts for all factors influencing Qfic, including RH and Tair; these latter parameters are often recorded in any case. This also means that this method can be subsequently applied to currently available data sets in order to improve their quality.Our results suggest that when the raw data are corrected appropriately, the SHB method is viable when attempting to determine transpiration rates of climbing plants. This is especially true for urban areas, with their illumination, typically high VPDs and increased Tair at night.