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

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 (èsoil)and stem water content (èstem)in the laboratory and field. Applications and advances of TDR and FDR in soil science have been welldocumented, but no attempt has been made to critically review the use of EM methods for measuring èstem. In this paper we review the principles, calibration, sensor configuration, and sensor installation of the TDR and FDR methods for the measurement of èstem. Perspectives are presented on new TDR calibration, the development of new sensors, and technologies for the simultaneous measurement of èstem, sap flow, bulk density/porosity, radial distribution of èstem, 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 èstem