Directional flow rate determination in trunks of apple trees in China’s Loess Mountain

Accurately measuring plant transpiration and water consumption is essential for regulating water and fertilizer use in traditional apple-fruit orchards. In this study, we combine thermal dissipation probes (TDP) and micro miniature automatic weather stations to evaluate the sap flow rate of apple-fruit trees planted in the north of Shaanxi on China's Loess Plateau, with the aim of improving accuracy of measurements to assist better decision making by orchard managers. TDP probes were inserted in different directions into the trunks of apple trees in their whole growth stages, and related factors were compared and analyzed. The results demonstrate that; 1) Sap flow rate (SFR) varies across the different growth stages of apple trees in a year; transpiration is lower during dormancy and maturity stages, and higher during periods of initial growth, rapid development and the Middle Growing Stage. However, SFR is higher during the day than at night, with average daytime transpiration accounting for over 86.29% of total transpiration. 2) SFR also varies depending on the direction of the TDP probe; determined SFR values were largest in the south, smallest in the north, and east and west values were approximately equal in the middle of the range. 3) The linear model built between SFR and transpiration indicates that both factors are closely correlated with east/west measurements. For east and west, the coefficients of determination are 0.74 and 0.83 and the square ratios are 78.21 and 137.85, respectively. Their correlation in the direction of both east and west is significantly better than in the north and south. 4) When water consumption is calculated according to the water balance equation, it is safe to draw the conclusion that transpiration approximately equals consumption when measured in the east/west direction. For east/west measurements, respective values for the mean square are 14.11 and 14.57, and significance levels are 0.020 and 0.019, which are both significantly better than values obtained from the north/south direction. Results from this study indicate that TDP probes should be inserted either in the east or west direction to effectively minimize error in the measurement of the sap flow.