A Method for In-Situ Measurement of Stem Water Content in Trees and Shrubs Using Time Domain Reflectometry

Abstract:

Quantifying vegetation water content is a critical aspect of understanding plant physiology, particularly how plants cope with drought, and ecosystem water balance. Yet, we lack a method to continuously monitor plant water content, particularly on small plants. We developed a method to continuously monitor tree and shrub water content using time domain reflectometry (TDR), a measurement technique commonly used to assess soil moisture. TDR probes were fabricated and inserted into trees and shrubs. Automated measurements were made at 30 minute intervals over several months. Calibration was performed by drying cut sections of trees and shrubs in the lab while making paired TDR and weight measurements on those samples to calculate gravimetric water content. Gravimetric water content was converted to volumetric water content to create a calibration equations relating TDR measurements to water content in Betula neoalaskana, Picea mariana, Populus tremuloides, and Salix alaxensis. Our fabricated TDR probes and our calibration equations permit continuous, non-destructive, and accurate measurements of stem water content in live trees and shrubs. These data show diurnal and seasonal patterns of water content which can be incorporated into plant physiological and hydrological models.