Abstract: Caragana microphyllais a typical sand-fixing species in Horqin sandy land. Using oxygen stable isotope technique, the relationships of δ¹⁸O of soil water, precipitation and xylem water were compared among 3-, 5- and 9-year-old C. microphylla individuals in the C. microphylla community during growing season. Water source of C. microphyllas with different tree ages was quantified using multiple linear mixing model. The water source of C. microphylla was analyzed according to the seasonal pattern of soil moisture and root distribution. By evaluating the adaptability of three aged C. microphylla individuals to regional water conditions, water competition of different aged individuals was analyzed to assess community stability. The results showed that root distribution determined the range of soil water use and water use strategy of different aged C. microphylla individuals, and soil water distribution also affected the main water sources of C. microphylla individuals. During the growing season, water in 30-80 cm soil layer was the main water source of three years old C. microphylla. The water in 30-100 cm soil layer was the main water source of five years old C. microphylla, with a larger contribution rate from 80-100 cm soil layer. For nine years old C. microphylla, 0-30 and 100-160 cm soil layers was the main water source. There was no strong water competition among different aged C. microphylla individuals, suggesting the community is stable.

Key words: riparian zone, niche breadth, niche overlap., niche similarity, Pterocarya stenoptera