Seasonal variation in δ 13 C of Pinus. yunnanensis and Pinus. armandii at different stand ages

Seasonal drought is common in Yunnan province, and water is the dominant factor limiting the growth of Pinus. yunnanensis and Pinus. armandii . The water use efficiency (WUE) of the two species is poorly understood. Needles were collected in a plantation ( P. yunnanensis and P. armandii mixed forest) in four seasons, and the needle δ 13 C values were measured. The selected species had larger δ 13 C values and exhibited higher WUE than typical subtropical species. P. armandii needles showed a more conservative water use strategy (high WUE) than P. yunnanensis . There were significant differences in the δ 13 C values of P. armandii between the two ages, whereas no difference was observed in the δ 13 C values of P. yunnanensis . The lowest δ 13 C value in the young P. armandii forest was observed in spring, whereas the δ 13 C value of middle-aged forests did not differ between the seasons. The δ 13 C value of young P. yunnanensis forests showed no difference in the four seasons, and the maximum value was observed in summer in middle-aged forests. In general, the δ 13 C value of P. armandii was lowest in spring, whereas that of P. yunnanensis was higher in spring and winter. The needle δ 13 C values were lower in spring and winter, indicating that the season had different effects on the δ 13 C values of different tree species. Correlation analysis between the needle δ 13 C values and meteorological data indicated that temperature and precipitation were the dominant factors affecting WUE in P. yunnanensis and P. armandii . The effect of temperature on WUE was greater in P. yunnanensis middle-aged forests. The identification and selection of subtropical tree species with high WUE are critical to maintaining high levels of forest benefits under limited water conditions.