Abstract: This study compared the water use strategy in 3, 5 and 9 year old Caragana microphylla communities in the Agula eco-hydrological research area. We compared, soil water, precipitation and xylem water content of C. microphyllas using oxygen stable isotope (δ¹⁸O) technology to examine their water utilization pattern in a period of drought followed by rainfall events (15.6, 33.1 mm) after 15 days. Root density and soil moisture dynamics were measured to help identify response mechanisms. The results show that, in drought, C. microphyllas mainly depended on maintaining internal moisture levels rather than absorbing subsoil water to survive. They may be utilized the condensation water of the surface soil; Within a day of rain, the δ¹⁸O content of xylem water at all ages trended to that present in the rain water. Within three days of rain, the δ¹⁸O of xylem water of 3 and 5 year plants changed remarkably, whereas 9 year plants remained stable. C. microphyllas of all ages mainly utilised soil water at 10 to 60 cm depth in the 5 days after rainfall; 3- and 5-year C. microphylla primarily utilised water from 1/3 point and edge, but the 9 year plants mainly used water from edge. With increasing tree age, the density of shallow roots increased markedly. It is suggested that manual control the distribution density of old C. microphyllas plants would help the hydrological balance in the region.

Key words: oxygen stable isotope, Caragana microphylla, growth years, water use strategy, density of roots