Abstract: To gain deep insights into shrub water use patterns and its response mechanism to environmental changes, it is essential to investigate the strategies of diurnal and nocturnal water use ratio by Salix psammophila under different moisture conditions. Three irrigation treatments were set in summer drought periods, including full irrigation (T₂₅, irrigated when the soil water potential at 20 cm directly below the dripper reached -25 kPa), water-controlled irrigation (T₅₀, irrigated when the soil water potential at 20 cm directly below the dripper reached -50 kPa), and no irrigation (CK, control). Environmental factors and the rate of sap flow in branches from different orientations were synchronously and continuously monitored. Plant water use was calculated based on the extension of branch base diameter in typical S. psammophila artificial forest of Hobq Desert. Results showed that there was a significant advantage in sap flow in the north direction of S. psammophila under T₂₅, and no significant advantage under other treatments. During the summer drought periods, the daytime water use (Q_d) and nocturnal water use (Q_n) of group T₂₅ increased by 255.6% and 72.7%, respectively, and the proportion of Q_n reduced from 4.0% to 2.0%. There was a significant positive correlation between the Q_d of T₂₅ and soil moisture (θ) of the 0-40 cm soil depth (P≤0.01). There was a significant negative correlation between Q_n and θ in the 40-60 cm soil layer (P≤0.05). The correlation between Q_d and Q_n in the controlled irrigation group and θ was not significant. The effects of vapor pressure deficit and relative air humidity on Q_d and Q_n decreased with increasing θ. Precipitation positively affected Q_n by increasing soil moisture (P≤0.05). We concluded that water use of S. psammophila can be derived well from the branch diameter, which was less affected by the branch orientation. The diurnal and nocturnal water use of S. psammophila was 0.27 to 0.96 mm·d^-1 and 0.009 to 0.019 mm·d^-1, respectively, indicating different adaptation to short-term and long-term soil moisture supply. Full irrigation did not alter the nocturnal water use habits of S. psammophila.

Key words: arid region, shrub, water use, soil moisture content, trade-off