Abstract: [Objective] This study investigated the characteristics of transpiration and the Tr/ET of the apple forests, and analyzed the relationship between transpiration of apple trees and environmental factors under different time scales, to provide a scientific basis for reducing the soil evaporation, controlling the transpiration and improving the efficiency of water use.[Method] This study was conducted in the Changwu Tableland, a typical representation of the Loess Plateau region. The thermal diffusion probe method was used to measure the sap flow of apple trees in 20a and 10a apple forests in 2015 at daily and monthly scales, and meteorological data were collected in an automatic meteorological observation station located at 50 m from the experimental plot, and at the same time, the soil moisture content and soil evaporation were determined.[Result] 1) On the day scale, there was a positive significant correlation between the transpiration of apple forests and air temperature (T_a) and VPD. However, the daily transpiration of the two apple forests declined when VPD> 1kPa. The monthly transpiration was also significantly correlated to air temperature (T_a) and VPD. 2) On the month scale, the experiment can be divided into three periods according to the soil water storage and consumption:the rapid soil moisture consumption period (May-Jul.), the soil moisture stable period (Aug.) and the slow soil moisture consumption period (Sep.-Oct.). Water consumption by transpiration of the 10-year-old apple forest mainly came from the soil layers of 0-300 cm, while the water consumption of the 20-year-old apple forest came from 0-600 cm. In the extremely dry month (Jul.), the water mainly came from the soil layers of 300-600 cm. 3) The Tr/ET of the 20-year-old apple forest was 29.25-67.51% and that of the 10-year-old apple forest was 36.44-62.06%. The Tr/ET of the different age apple forests showed the trend that increased first and then decreased. The Tr/ET of 20-year-old apple forest declined sharply after reaching a peak in August, while the 10-year-old apple forest declined sharply in September after a transition period.[Conclusion] With the increase of time scales, the number of environmental factors associated with the sap flow velocity was gradually reduced, but VPD was the dominant factor at each time scale. The water source of transpiration of the 10-year-old apple tree was mainly concentrated in the 0-300 cm soil layers, while that of the 20-year-old apple tree was from the 0-600 cm soil layers. In the extreme water deficit July, the more water came from 300-600 cm soil layers in the 20-year-old apple forest. After the end of August, some appropriate conservation measures on soil moisture should be taken to reduce evaporation for the 20-year-old apple forest, while the corresponding measures can be carried out in September for the 10-year-old forest. Those measures would provide a relatively sufficient soil moisture for healthy growth of apple trees in the next year.

Key words: TDP(thermal dissipation probe), transpiration, environmental factor, Tr/ET