Objective  Quercus variabilis is one of the dominant tree species in Beijing mountainous area. To explore the water use strategies, sap flow characteristics and their relationships of Q. variabilis could provide a theoretical reference for the study of water sorption process of trees, and enhance the knowledge of forest plantation management.

  Method  Based on the stable hydrogen and oxygen isotopes and the principle of thermal diffusion (pin type) stalk flow meter, water use sources and sap flow rates of Q. variabilis were measured in different seasons from 2015 to 2017, and the correlation between the two indicators was analyzed combined with meteorological data.

  Result  (1) There were significant differences in sap flow activities in different stages of the non-growing season (November to December and January to March). The average daily sap flow rate gradually increased by the end of the dormant period, and the sap flow rate was 2 × 10⁻⁴−3 × 10⁻⁴ cm/s, which was affected by environmental factors in the growing season with a large daily fluctuation of 1.5 × 10⁻³ to 1.7 × 10⁻³ cm/s. During the observation period, the sap flow rate differed significantly from June to October, but the difference from April to May was insignificant in different years; (2) the ratios of water use by Q. variabilis in different soil depths were relatively stable among different seasons, and the water in each soil layer was evenly supplied to the tree. In the non-growing season, the absorption and utilization ratios of water from the 0− 40 cm, 40−80 cm, and 80−100 cm soil layers by Q. variabilis were (36.0 ± 3.5)%, (41.0 ± 1.6)%, and (23.0 ± 2.3)%, respectively. During the growing season, the utilization of soil water of 0−40 cm depth increased to (39.3 ± 2.6)%. The ratio of soil water utilization from 40−80 cm remained unchanged (40.0 ± 1.5)%, and soil water utilization of 80−100 cm decreased to (20.7 ± 1.8)%; (3) the average daily transpiration was negatively correlated with the use of 40−80 cm soil water by Q. variabilis during the growing season and was positively correlated with the use of 80−100 cm soil water.

  Conclusion  Under the existing seasonal water use pattern of Q. variabilis, the increase in transpiration during the growing season will expand the absorption ratio of deep soil water, which is of great significance for the maintenance of transpiration of Q. variabilis.