Monitoring of Stem Water Content of Native and Invasive Trees in Arid Environments Using GS3 Soil Moisture Sensors

Dielectric soil moisture sensors have the potential for nondestructive and real-time monitoring of the stem water content (θ st ) of living trees. This study was conducted to investigate the water use characteristics of trees in drylands through monitoring of θ st using newly developed capacitance sensors (GS3). The plants used for data collection were Prosopis juliflora (Sw.) DC. (mesquite, invasive) in Sudan and Tamarix ramosissima Ledeb. (tamarisk, invasive) and Prosopis pubescens Benth.(screwbean mesquite, native) in the United States. The GS3 probes were installed into the trunks of two trees for each species. Stem-specific calibration equations and temperature calibration equations were derived through laboratory experiments and analysis of field observation data. The temperature calibration equations reduced inappropriate variations of θ st caused by daily fluctuations in stem temperature, suggesting that these are essential for correct interpretation of monitoring data of θ st in arid environments. The θ st of the mesquite trees in Sudan clearly increased after heavy rainfall events and started decreasing when the soil water content became close to the wilting point. These findings indicate that mesquite trees use soil water in rainy seasons, even though they are generally considered to use groundwater through deep tap roots. The θ st of neither species in the United States responded clearly to rainfall events, indicating that they depend on shallow saline groundwater. The θ st of the tamarisk decreased monotonically throughout the monitoring period, apparently in response to feeding damage caused by the tamarisk leaf beetle ( Diohabda sp.), which had been released for biological control of tamarisk.