Abstract
Dielectric soil moisture sensors are commercially available and widely used in soil science. In this study a type of soil moisture sensor was applied to the moisture measurement of living and standing conifer tree stems. Some noticeable seasonal and diurnal changes of volumetric water content (VWC) of stem were observed during a long-term continuous measurement with high time-resolution. A detailed analysis revealed the following four kinds of factors influencing the changes of the measured VWC of tree stems. (1) Gradual decrease of measured VWC for one to two months starting at sensor installation due to dehydration from xylem surrounding the sensor probes. (2) Rapid decrease of measured VWC due to winter stem freezing. (3) Rapid increase or decrease of measured VWC after rainfall due to electric noises in the measurement system. (4) Temperature dependency of measured VWC on stem temperature, which arose from the dependency of water permittivity on temperature. Factors (1) to (3) above are unavoidable in the field measurement and uncontrollable by operation; the solution is data omission in the analyses of stem moisture fluctuation under conditions when such factors arise. The factor (4) can be compensated with the simultaneous temperature measurement of stems, by a theoretical correction from the relationship between temperature and water permittivity, and by an empirical correction developed by data analyses of the observed relationship between stem temperature and measured VWC. Corrected data showed that the seasonal fluctuation of stem VWC was small in general, in agreement with the results from former destructive and non-destructive methods, but sometimes the VWC was apparently increases or decreases and that the fluctuating patterns differed between species, between individuals and even between positions within a tree.
