Seasonal drought is common in Yunnan province, and water is the dominant factor limiting the growth ofPinus. yunnanensisandPinus. armandii. The water use efficiency (WUE) of the two species is poorly understood. Needles were collected in a plantation (P. yunnanensisandP. armandiimixed forest) in four seasons, and the needle δ13C values were measured. The selected species had larger δ13C values and exhibited higher WUE than typical subtropical species.P. armandiineedles showed a more conservative water use strategy (high WUE) thanP. yunnanensis. There were significant differences in the δ13C values ofP. armandiibetween the two ages, whereas no difference was observed in the δ13C values ofP. yunnanensis. The lowest δ13C value in the youngP. armandiiforest was observed in spring, whereas the δ13C value of middle-aged forests did not differ between the seasons. The δ13C value of youngP. yunnanensisforests showed no difference in the four seasons, and the maximum value was observed in summer in middle-aged forests. In general, the δ13C value ofP. armandiiwas lowest in spring, whereas that ofP. yunnanensiswas higher in spring and winter. The needle δ13C values were lower in spring and winter, indicating that the season had different effects on the δ13C values of different tree species. Correlation analysis between the needle δ13C values and meteorological data indicated that temperature and precipitation were the dominant factors affecting WUE inP. yunnanensisandP. armandii. The effect of temperature on WUE was greater inP. yunnanensismiddle-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.