Abstract: In the context of water scarcity, understanding the water sources of major afforestation species would provide a reference for urban greening and water resource management in arid oasis areas. Taking three key tree species of afforestation, Platanus orientalis, Ulmus pumila, and Robinia pseudoacacia, in the Kashgar region on the western edge of Tarim Basin as objects, we analyzed the stable oxygen isotope values (δ¹⁸O) of xylem water from branches and their potential water sources, along with stable carbon isotope values (δ¹³C) of leaves, to determine the proportion of different water sources for each species and assess their water use efficiency (WUE). The results showed that P. orientalis exhibited diverse water source utilization, with a relative balance distribution among groundwater, river water, and soil water at varying depths (23.0% of groundwater, 20.7% of river water, 19.0% of depth soil water 0-60 cm, 19.6% of depth soil water 200-300 cm). In contrast, U. pumila and R. pseudoacacia relied more on soil water. U. pumila accessed soil water at depth of 0-60 cm (55.2%), while R. pseudoacacia utilized water from the depth of 0-140 cm (61.3%). The foliar δ¹³C values of R. pseudoacacia (26.0‰) were higher than that of P. orientalis (28.0‰) and U. pumila (27.4‰), indicating higher WUE. Under the drought condition, P. orientalis exhibited resilience by using soil water, groundwater, and river water. In contrast, U. pumila and R. pseudoacacia were constrained to soil water and had overlapping water absorption depths. It is necessary to avoid water competition between the two species in mixed planatation.

Key words: arid oasis region, greening tree species, water source, δ¹⁸O, δ¹³C, water use efficiency