Abstract
Study region: This study was conducted at Xinhua Forest Farm, located in the desert–oasis transition zone of the Ulan Buh Desert, northwestern China, where mean annual precipitation and evaporation are approximately 132 and 2032 mm, respectively. Sap flow of Pinus sylvestris var. mongolica, Ulmus pumila, and Elaeagnus angustifolia was monitored during the 2024–2025 growing seasons. Study focus: Generalized additive mixed models and distributed lag non-linear models were used to quantify nonlinear sap-flow responses to solar radiation (SR), vapor pressure deficit (VPD), and soil water content (SWC), and to assess lagged rainfall effects under different antecedent soil-moisture conditions. New hydrological insights for the region: Cumulative sap flow was 1189.44, 1313.75, and 1849.44 kg for P. sylvestris, U. pumila, and E. angustifolia, respectively. Sap flow responded nonlinearly to all three environmental drivers (P < 0.001). The SR turning points were 323.0, 323.0, and 333.1 W m⁻²; the VPD turning points were 2.814, 1.763, and 2.223 kPa; and the SWC turning points were 0.0810, 0.0968, and 0.0586 m³ m⁻³ , respectively. Post-rainfall responses lasted 2–4 h in P. sylvestris, approximately 10 h in U. pumila, and more than 15 h in E. angustifolia. Their magnitude and duration depended strongly on antecedent SWC. These findings show that rainfall use by shelterbelt trees is jointly regulated by species-specific environmental thresholds and antecedent soil moisture.