CO_2 efflux (E_(CO_2)) from stems and branches is important in the regulation of biomass productivity and maintenance of carbon balance in terrestrial ecosystems.E_(CO_2) is a temperature-sensitive physiological process.An increase in temperature could promote the activities of enzymes,decrease the gas solubility,and improve the diffusion coefficient of CO_2.Until recently,great uncertainties relevant to the variations in E_(CO_2) and their temperature-sensitivity (expressed in terms of Q_(10)) remain poorly assessed because of the lack of continuous sampling,both in time series and space sequence.Understanding the effect of temperature on E_(CO_2) at different time scales is important to accurately upscale the chamber-based CO_2 measurements to the whole-stem.To investigate the temporal dynamics and vertical variations in woody-tissue E_(CO_2) and their temperature sensitivity,the hourly E_(CO_2) at four different heights (10,130,240,and 370 cm) of the four typical tree species (Sophora japonica,Salix matsudana,Larix principis-rupprechtii,and Platycladus orientalis) was observed using an Li-8100 automatic instrument (LI-Cor,Inc,NE,USA) on two consecutive days per month throughout 2014 in an artificial mixed forest plantation near Beijing.Simultaneously,the meteorological conditions (air temperature,relative humidity,and woody-tissue temperature) and stem radial increment were monitored using the HOBO Pro V2 probes (Onset Computer Corporation,Bourne,MA,USA) and dendrometer (CR10X,Campbell Scientific,Logan,UT,USA),respectively.The results indicated that: (1) the E_(CO_2) of the four tree species showed similar seasonal variations,characterized by a significantly higher E_(CO_2) in the growing months than in the dormant months.Together,the woody-tissue temperature (T_W) and stem radial diameter increment (Di) of the trees explained the seasonal variation of E_(CO_2);(2) Our study showed that the temperature-sensitivity coefficient,Q_(10),was not a constant,as assumed in some models,but was instead highly dependent on the measurement temperature.A lower Q_(10) was observed in the summer months,but a higher value was observed in the dormant months.A strong vertical variation in Q_(10) was observed during the growing season,showing a gradient increment with height along the trunk,and the vertical variation disappeared in the dormant months;(3) Based on the strength of correlations between stem (T_W) and air temperature (T_A),the daytime was divided into four periods.In growing months,the night-time temperature-sensitivity coefficient Q_(10) was significantly higher than that of day-time values,especially for the two deciduous species (Sophora japonica and Salix matsudana).This result implies that E_(CO_2) is more sensitive to temperature in the night than in the daytime,and the temperature responses of E_(CO_2) estimated by only daytime measurement can lead to the underestimated stem respiration,especially considering that the temperature increase is faster during the night-time.In contrast,the diurnal asymmetry of Q_(10) disappeared in the dormant months,i.e.,no significant difference was observed between each of the four periods.The quantitative analysis of the temporal dynamics and vertical variations of E_(CO_2),as well as what factors are driving these changes,would help us to improve our knowledge of the E_(CO_2) processes and understand how climatic changes affect forest carbon budgets.