Dieback of Mongolian pine (Pinus sylvestris var. mongolica) wide windbreaks often emerges 35 years after planting in agroforestry systems. Water stress is considered to be the main cause for such dieback, but there is little knowledge about how water use efficiency of trees changes with their age. We measured needle δ13C as a proxy for intrinsic water use efficiency (iWUE), combined with specific leaf area, leaf dry matter content, soil water content, and groundwater level in 9‐, 21‐, and 41‐year‐old pine stands in 2011. Results showed that the mean δ13C value of 9‐year‐old trees was lower by 0.66 and 0.48 ‰ than that of 21‐ and 41‐year‐old trees, respectively, but the mean δ13C value in 21‐year‐old trees was higher by 0.17 ‰ than that in 41‐year‐old trees. Soil water content for 9‐year‐old trees was highest. Specific leaf area for 21‐year‐old trees was lowest. Compared with the other two tree ages, tree roots reached groundwater levels for 41‐year‐old trees. These results suggested that (1) 9‐year‐old trees with the lowest iWUE applied a prodigal water use strategy due to higher soil moisture, (2) 21‐year‐old trees with the highest iWUE employed a more conservative water use strategy and experienced significant water stress because of lower soil moisture and specific leaf area, and (3) the water use strategy of 41‐year‐old trees was between that of 9‐ and 21‐year‐old trees, likely due to both aging and uptake of groundwater. Dieback of Mongolian pine trees at older age (e.g., more than 35 years old) might occur when the groundwater level declines suddenly in years of extremely low rainfall.
