Abstract: Clarifying water sources and competition between evergreen and deciduous trees can provide a theoretical basis for the selection and planting methods of tree species during ecological restoration. We measured the ratio of hydrogen-oxygen stable isotopes of soil, precipitation, and xylem of Platycladus orientalis and Ulmus pumila in the Nanshan Greening Engineering Area of Lanzhou. The characteristics of soil water utilization in different depths were analyzed by MixSIAR and proportional similarity index. The results showed that except for July and October, deep (70-100 cm) soil water was the main water source of P. orientalis and U. pumila, with a contribution rate of 55.82% and 58.90%, respectively. In July, both species absorbed shallow (0-20 cm) soil water, with a contribution rate of more than 90%. In October, P. orientalis absorbed middle-layer (20-70 cm) soil water, while U. pumila used both middle-layer (20-70 cm) and deep (70-100 cm) soil water. In summary, P. orientalis and U. pumila do not compete for water in the growing season except July. They can be planted simultaneously in the same habitat.

Key words: hydrogen and oxygen stable isotopes, MixSIAR, proportional similarity index, Platycladus orientalis, Ulmus pumila, water source