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Mongolian pine (Pinus sylvestris var. mongolica) plantations in semiarid sandy lands play an important role in the prevention and control of desertification in northern China, but plantation dieback often occurs approximately 30-35 years after planting due to water deficiency. However, the seasonal changes in the water sources used by Mongolian pine trees of different ages remain unclear, and these changes influence the management of the plantations. During two consecutive years, 2012 and 2013, stable isotopes of hydrogen and oxygen from twig xylem water, soil water at different depths (0-20, 20-40, 4060 and 60-100 cm), precipitation and groundwater were analyzed in 10-, 22-, 32- and 42-year-old Mongolian pine plantations to identify the sources of water used by the trees during different seasons (spring, summer and autumn). The soil water content, groundwater level and delta C-13 of the needles were measured concurrently, and the results showed that the 10- and 22-year-old trees only used soil water during the two years of observation, but the 32- and 42-year-old trees utilized both soil water and groundwater. In spring (late May), when soil water content was low, the 10- and 22-year-old trees used water from the 20-60 cm soil layer, but the 32- and 42-year-old trees utilized water from the 40-100 cm soil layer and groundwater. In summer (mid or late July), when soil water content was high, the trees of different ages used water from the 0-60 cm soil layer, and in autumn (mid or late September), when the soil water content was relatively high (2012), the trees of different ages used water from the 20-100 cm soil layer. However, when the soil water content was relatively low (2013), the 10- and 22-year-old trees continued to use water from the 20-100 cm soil layer, but the 32- and 42-year-old trees absorbed water from both the 60-100 cm soil layer and the groundwater. The contribution of groundwater to tree transpiration increased with decreasing soil moisture for the 32- and 42-year-old trees, and groundwater was the dominant water source (e.g., 60.2% of tree transpiration in the 42-year-old trees) when the soil moisture was approximately 20% of field capacity (3.5%). In addition, the significantly higher delta C-13 in the needles of the 22-year-old trees relative to the four ages of pine trees suggested higher water use efficiency in the 22-year-old trees. These findings suggest that the old Mongolian pine plantations (e.g., 30-35 years old) will experience greater mortality in the future if the groundwater level suddenly declines under drought conditions, but the young Mongolian pine plantations would suffer only slightly because their roots are unable to reach the saturated zone. (C) 2016 Elsevier B.V. All rights reserved.