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Adaptive mechanisms in Hedysarum mongolicum to dry and hot conditions in desert areas remain unclear. In this work, the diurnal and seasonal dynamics of sap flow in H. mongolicum, endemic to a semi-arid desert shrubland ecosystem, were studied. Stem sap flow was continuously monitored with heat balance sensors from June to October, 2012. Diurnal pattern in sap flow per stem area (J(s)) revealed a clear seasonal trend, with J(s) peaking earlier in summer (similar to 10:00 h, 3 h before solar noon, R-s), than in autumn (at roughly the same time as R-s). Seasonally, J(s) was positively correlated to leaf area index. Excluding effects of phenology, volumetric soil water content (VWC) affected J(s) the greatest. In general, VWC was revealed to regulate the degree of response of J(s) to meteorological factors [R-s, temperature T, and water vapor pressure deficit (VPD)], with sap flow being more responsive to these factors when VWC was high ([0.09 m(3) m(-3)). Under low VWC (<0.09 m(3) m(-3)), positive response of J(s) to the meteorological factors became saturated (leveled off) or declined when dry-hot conditions exceeded an hourly mean T, R-s, and VPD of 24 degrees C, 700 W m(-2), and 2.1 kPa, respectively. The results showed that H. mongolicum shrubs adapt to hot-dry or water-limited conditions in summer by closing their stomata in mid-morning and reducing the sensitivity in J(s) to T, R-s, and VPD. The understanding of adaptive mechanisms in desert shrub species could potentially provide important baseline information towards the ecological restoration and sustainable development of arid-and semi-arid ecosystems.