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

Time Domain Reflectometry (TDR) has previously been used todetermine the water content of soils. Here, TDR is assessedas a method of tracking the seasonal change in water contentof the stems of mature trees (Pinus sylvestris L.). The longerprobes used for soil were replaced by 50 mm probes, insertedradially into the stems at 1 m above the ground. Tests on blocksof wood in the laboratory suggested that the probe is influencedby the water content several centimetres around it, and thesensitivity of the system enables volumetric water content (W)in the physiological range to be measured to a resolution greaterthan 0.01 m³ (H₂0) m^–3. The use of very short probes preventsthe development of a universal calibration between measureddielectric constant and W. Calibrations for both 50 mm and 20mm long probes are reported. The effect of temperature on thecalibration was negligible. The system was used successfullyon mature trees in 1995, and no evidence of wound reaction aroundthe permanently installed probes was apparent in the measurements. Key words: TDR, dielectnc constant, water content, Pinus sylvestris, xylem embolism