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Natural Mongolian pine trees of different ages consistently use shallow water throughout the main growing season; therefore, water stored in the shallow soil layer is vital for maintaining their viability.

Mongolian pine (Pinus sylvestris var. mongolica) plantations in sandy regions often experience dieback after 30-35 years of growth due to water deficiency, whereas natural Mongolian pine forests remain healthy during the same growth stage. However, the water use strategies of natural Mongolian pines remains unclear. Therefore, delta H-2 and delta O-18 in twig xylem water, soil water and groundwater were analyzed in 10-20, 20-30 and 30-50-year-old natural Mongolian pine trees to identify their water sources. In addition, needle delta C-13 was measured simultaneously to assess water use efficiency. Results showed that pine trees of different ages utilized soil water from the same depth. During the growing season (June-August), all pine trees utilized water from 0 to 20 cm soil depth, regardless of the soil water condition. During the end of growing season (September and October), even though soil moisture content in the 0-20 cm depth was higher, pine trees of different ages utilized water from the 0-60 cm soil depth in September and switched to utilize water from the 20-80 cm soil depth in October. There were no significant differences in needle delta C-13 among the sampling dates for trees in each age group, indicating that pine trees can absorb sufficient water to satisfy their water requirements regardless of age. These findings suggest that water stored in the shallow soil layer (0-20 cm) plays an important role in supporting tree transpiration during the growing season (June-August). Therefore, the stability of shallow soil is vital for maintaining the viability of natural Mongolian pine forests.