SREL Reprint #3624

Recalibrating best practices, challenges, and limitations of estimating tree transpiration via sap flow

Mackenzie J. Dix1,2 and Doug P. Aubrey1,2

1Warnell School of Forestry and Natural Resources, University of Georgia, Athens, GA, USA
2Savannah River Ecology Laboratory, University of Georgia, Aiken, SC, USA

Abstract:
Purpose: Transpiration dominates hydrologic fluxes from terrestrial ecosystems to the atmosphere. Sap flow is the most widely used method to quantify transpiration, and calibrations can improve the accuracy of these estimates. Despite numerous suggestions to perform and apply sap flow calibrations, seemingly few reports apply these recommendations. Here, we sought to determine the prevalence of calibration across sap flow methodology.
Recent Findings: We reviewed tree sap flow literature over the past 9 years and determined that only 5.3% of these publications documented the performance of calibrations, the application of coefficients from previous calibrations, or the application of correction factors using calibration data to ensure or improve accuracy of transpiration estimates. These data indicate that calibration has not been adopted as a best practice. However, these data may also reflect challenges to the performance and application of calibration indicating fundamental limitations of sap flow methods.
Summary: We highlight the need for testing critical assumptions regarding calibration; specifically, that coefficients derived from calibrations on small stems are suitable for application to large stems and that different calibration approaches yield similar transpiration estimates. We encourage researchers to adopt calibration as a best practice for sap flow data and to explain how calibration was performed, how calibration data were applied, and to acknowledge and justify circumstances that preclude or do not necessitate calibration. Without the adoption of calibration and an improved understanding of its limitations, our understanding of transpiration and ability to quantify it using this technology will remain inadequate.

Keywords: Calibration, Ecohydrology, Transpiration, Sap flux, Water use

SREL Reprint #3624

Dix, M. J., and D. P. Aubrey. 2021. Recalibrating best practices, challenges, and limitations of estimating tree transpiration via sap flow. Current Forestry Reports 7(1): 31-37.

This information was provided by the University of Georgia's Savannah River Ecology Laboratory (srel.uga.edu).