Time and frequency domain reflectometry for the measurement of tree stem water content: A review, evaluation, and future perspectives

Abstract

Electromagnetic (EM) methods for estimation of water content in porous media (e.g., soil and tree stems) are based on estimates of the media's bulk dielectric permittivity, which is sensitive to water content. Time domain reflectometry (TDR) and frequency domain reflectometry (FDR) are routinely used for rapid, automated and minimally destructive approach for routine measurement of soil water content (θsoil) and stem water content (θstem) in the laboratory and field. Applications and advances of TDR and FDR in soil science have been well-documented, but no attempt has been made to critically review the use of EM methods for measuring θstem. In this paper we review the principles, calibration, sensor configuration, and sensor installation of the TDR and FDR methods for the measurement of θstem. Perspectives are presented on new TDR calibration, the development of new sensors, and technologies for the simultaneous measurement of θstem, sap flow, bulk density/porosity, radial distribution of θstem, and electric conductivity. Also discussed were the effects of tree morphology and wounding on the accuracy of both methods. This review provides information for the novice and expert alike to guide them on the advantages, limitations, development, and application of the EM methods required to improve the reliability and validity of measured values of θstem.