Abstract:According to a mass balance approach, plant stem respiration(RS) is composed of three parts that accounts for all fluxes of carbon dioxide of stems: transport flux (FT , dissolved CO2 entering and leaving the segment in flowing sap); storage flux (△S, the increase or decrease in mean sap ［CO2］ over time); efflux to atmosphere (EA, CO2 leaving the segment by diffusion through bark), in which stem internal CO2 transport in xylem sap flow (FT) plays a very important role in gas exchange of plant. It is an essential and non-specific component in respiration measurements of stems and branches. To evaluate the contribution of these three flux components to the stem respiration of Larix gmelinii, the stem CO2 efflux was continuously determined using infrared gas analysis method (IRGA) in situ and the sap flow density and stem temperature were simultaneously recorded. The relationship between stem respiration and stem temperature was fitted by Arrhenius equation assumed that the CO2 efflux is equal to the stem respiration rate when the sap flow ceased. Then, the stem respiration rate (RS), CO2 transport flux in sap flow (FT) and storage flux (△S) were calculated according to mass balance method. Our results showed that proportions of EA, FT and △S accountting for RS are dynamic. CO2 efflux (EA) on the stem surface closely related with internal CO2 flux of the stem. It means that FT and △S can affect EA. In 24 h cycle, EA, FT, △S accounted for 65.10%~104.45%, 1.86%~29.46%, and 0.42%~5.44% of the total stem respiration, respectively. Sap flow is the main factor affecting the composition of the three flux components, but the effect is different among the individual trees.