Duration of Available Sap Flux Density Data Measured by Sensors Applied to Japanese Cedar

Abstract

　In order to avoid underestimations by methods of measuring sap flux densities (SFD), previous studies have suggested that sensors are replaced every growing season and/or year. However, limited information is available showing how the underestimations can be detected. We validated the detections of underestimations based on the relationship between the vapor pressure deficit (VPD) and SFD for data measured in stands of Japanese cedar (Cryptomeria japonica D. Don) planted in the Tsukuba Experimental Watershed (TEW) located in the Kanto district, and the Nagasaka Experimental Watershed (NEW) located in the Tohoku district, in Japan. The detections were valid for the data measured with the Granier method, the heat ratio (HR) method, and the heat field deformation (HFD) method. Decrease in the water content of sapwood around the sensors was determined as the cause of the underestimations and implied the sap flow encircling the inserted sensors. The sensors, durations of available data of which were 365 days or less, were 62 % and 86 % of 52 Granier sensors in TEW and 64 sensors in NEW, respectively. For measurements taken year-round, we suggest that researchers accept the possibility of replacing sensors within a year after installation and consider designing the measurements accordingly.