Development of a stem water storage detection sensor using the dielectric properties of living standing trees

Stem water content (StWC=volume of water: volume of the stem) has been another important physiological parameter of living trees, in addition to the sap flow. An accurate StWC can greatly contribute to evaluating the resistance of drought and/or cold, precise irrigation scheduling, and health development in forestry. Therefore, stable, reliable, and convenient non-destructive sensors have been widely applied to detect the StWC, in order to reduce the damage to the tree growth in the process. In this study, a combined sensor was designed to simultaneously detect the stem moisture and pressure between probe and stem using the dielectric characteristics of living trees and multi-sensor fusion technology, in order to deal with the fixed probe size limiting the radial growth of the stem. The accuracy and practicability of the living tree StWC sensor were improved using the Standing Wave Rate (SWR) in the current non-destructive detection, further reducingthe measurement error caused by the contact pressure between the probe and stem with the stem diameter fluctuations. The sensor included the arc moisture probe, pressure sensor, fixed rope, and transmission device. A single-chip microcomputer was used as the controller to tune the retraction and release of fixed rope for the pressure stability during detecting. At the same time, the moisture probe was close to the stem, where a coplanar electrode was arranged on the same side of the stem up and down, rather than a traditional parallel plate electrode. A Honeywell Fire Software Suite (HFSS) was used to simulate the electric field distribution around different arc probes, thereby finally determining the structure size. Since the radial detection distance limit of the sensor was 30 mm, the most sensitive distance was 0-10 mm, indicating the uneven sensitivity in the radial direction. A field experiment was also conducted to measure the dynamic and static characteristics of the sensor. The organic solution with different dielectric constants was used to simulate the stem tissue with different moisture contents. A moisture detection unit of the sensor presented an excellent linear relationship in the dielectric constant range of 6-53.3 (20 ℃) (R2=0.990 5), and the measurement range of stem moisture content was 0-85%. In addition, the poplar sawdust and water were mixed in different proportions to prepare the samples with the moisture contents (0, 9%, 17%, 29%, and 100%), where the sensor was continuously measured for 300 min. The sensor presented excellent output stability, where the maximum fluctuation in the test time was 0.3% Full Scale (FS). A cylindrical device was specially designed to expand the diameter by rotating the rotary table. As such, the radial growth of the stem was simulated to observe the adjustment of the sensor. It was found that the moisture measurement output was significantly reduced the fluctuation, where the sensor always remained the pressure within the preset range during the experiment. Consequently, the stem segment of healthy living poplar (diameter 5.9 cm, length 9.4 cm) was intercepted for five consecutive measurements, indicating the better repeatability of the sensor than before. Subsequently, two sensors were then installed on the stem of the poplar planted in the greenhouse (50 cm from the ground) for six consecutive days, and the data was collected every 30 min by the data collector. It was found that the sensor shared the better adaptability to the change of stem diameter, indicating better performance. This finding can also provide new equipment to assess the tree tending and healthy growth in forestry.