【Objective】This study aimed to investigate the mechanism of photosynthate supply on stem CO_2 efflux and its temperature sensitivity of tree trunk surface,which is not only critical for understanding the carbon cycling processes of forest ecosystem,but also provides theoretical basis for forest and plantation management under global climate change. 【Method】In this study,10-year-old Populus tomentosa plantation was taken as the research object,and a random block experiment was conducted with three plots (30 m * 30 m) in Chinese white poplar (P. tomentosa) plantation in northern China. A girdling experiment on the tree stems was operated to change the supply of photosynthate. The stem CO_2 efflux (E_s) and stem temperature (T_(stem)) of Chinese white poplar were measured to examine the effects of photosynthates supply on the temperature sensitivity (Q_(10)) of E_s above girdle (AG) and below girdle (BG). Meanwhile,the dynamic changes of nonstructural carbohydrates in stem tissue were measured for comparing different responses of E_s and Q_(10) to substrate supply in growing and non-growing seasons.【Result】1) E_s of AG was increased by 57% and E_s of BG was decreased by 43% in growing season after the 30-day girdling treatment,whereas no obvious difference in E_s was detected among NG,AG,and BG in non-growing season. 2) Soluble sugar contents of the AG and BG in growing season were decreased by 29% and 15%,respectively. Similarly,the soluble sugar content of AG was also decreased by 15%,but the soluble sugar content of BG was increased by 10% in non-growing season. 3) We also found exponential relationships between E_s and stem temperature in both growing and non-growing seasons,but the R squares (R~2) of the exponential relationship between T_(stem) and E_s at AG and BG were decreased under stem girdling treatment. 4) Both the Q_(10) and stem basal respiration rate (R_(15)) at AG were increased,while those of BG were decreased under stem girdling treatment.【Conclusion】Stem girdling blocked the import of photosynthates,and thus altered the soluble sugar contents of AG and BG. Stem girdling resulted in increases in the E_s and Q_(10) at AG,but led to decreases in the E_s and Q_(10) at BG. The responses of stem CO_2 efflux and its temperature sensitivity to stem girdling were obviously different between growing season and non-growing season.