Abstract: [Objective] Helianthemum songaricum is a rare and endangered relict perennial xeric shrubs in western Ordos. Water plays an important role in the growth and function of desert plants,and precipitation is the main water source in arid and semiarid regions. Therefore researches on the water use patterns of desert plants under different intensities of rain events are helpful to understand the growth regulation and distribution tendency of plants and their response mechanism to rainfall.[Method] To quantify the utilization of rains with different pulse intensities by H. songaricum and Sarcozygium xanthoxylum, we established and analyzed relationships between plant xylem water of H. songaricum and S. xanthoxylum and potential water (precipitation, soil water and ground water) within 9 days following each natural rain eventusing hydrogen stable isotopes.[Result] 1) We found that the soil moisture increased and soil water δ Ddecreased in the surface soil of 0-20 cm after 8.6 mm within 9 days, and the soil moisture and soil water δ Dchanged obviously in the soil of 0-40 cm after 12.1 mm within 9 days, whereas the soil moisture and soil water δ Dchanged significantly in all soil depth after 79.6 mm with in 9 days. 2) For the three rain events observed, the utilization rate of rain water by H. songaricum and S. xanthoxylum within 9 days was the highest under the large rain event (>20mm) at 76.4%-98.5% and 55.6%-74.3% respectively. The highest utilization rate of rain water by H. songaricum within 9 days was 67.0% under the small rain event(<10 mm)and 71.8%under the intermediate event.It is indicated that H. songaricum could use the limited rain water effectively, which could greatly improve its chance for survival. 3) The utilization rate of rain water by H. songaricum was significantly higher than that by S. xanthoxylum in despite of the intensities of rain events.4) In the intermediate rain event, the competition between H. songaricum and S. xanthoxylum for the soil water was obvious.However in the small rain event,no competition between the two species was observed in the same vegetation communities as they were dependent on soil water in different layers.5) The root of H. songaricum was mainly distributed in soil layers of 0-40 cm, its fine roots are mostly distributed in the layers of 0-20 cm and 20-40 cm, which accounted for 58% and 37% of the total fine root biomass respectively, while the root biomass of S. xanthoxylum was primarily distributed in soil layers of 20-60 cm, its fine roots were mostly distributed in the layers of 20~60 cm (68.2%).[Conclusion] The large rain event (>20 mm) affected the soil moisture and soil water δ D in all soil layers, whereas the intermediate rain event (10-20 mm) displayed influences in the soil layers of 0-40 cm and the small rain event (≤10 mm) only in the surface soil layers of 0-20 cm.H. songaricum could use the rain water effectively under different invensity rainfalls, but S. xanthoxylum only depended on heavy rain.The water use patterns of H. songaricum and S. xanthoxylum were consistent with the distribution patterns of their root systems.

Key words: hydrogen stable isotopes, Helianthemum songaricum, Sarcozygium xanthoxylum, water use, Western Ordos