Non-destructive measurement of stem water content by time domain reflectometry using short probes

Abstract

Time Domain Reflectometry (TDR) has previously been used to determine the water content of soils. Here, TDR is assessed as a method of tracking the seasonal change in water content of the stems of mature trees (Pinus sylvestris L.). The longer probes used for soil were replaced by 50 mm probes, inserted radially into the stems at 1 m above the ground. Tests on blocks of wood in the laboratory suggested that the probe is influenced by the water content several centimetres around it, and the sensitivity of the system enables volumetric water content (W) in the physiological range to be measured to a resolution greater than 0.01 m3 (H20) m-3. The use of very short probes prevents the development of a universal calibration between measured dielectric constant and W. Calibrations for both 50 mm and 20 mm long probes are reported. The effect of temperature on the calibration was negligible. The system was used successfully on mature trees in 1995, and no evidence of wound reaction around the permanently installed probes was apparent in the measurements.