Seasonal variation in δC 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 δC values were measured. The selected species had larger δC 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 δC values of P. armandii between the two ages, whereas no difference was observed in the δC values of P. yunnanensis. The lowest δC value in the young P. armandii forest was observed in spring, whereas the δC value of middle-aged forests did not differ between the seasons. The δC 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 δC value of P. armandii was lowest in spring, whereas that of P. yunnanensis was higher in spring and winter. The needle δC values were lower in spring and winter, indicating that the season had different effects on the δC values of different tree species. Correlation analysis between the needle δC 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.