Abstract: According to a mass balance approach, plant stem respiration(R_S) is composed of three parts that accounts for all fluxes of carbon dioxide of stems: transport flux (F_T , 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 (E_A, CO₂ leaving the segment by diffusion through bark), in which stem internal CO₂ transport in xylem sap flow (F_T) 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 (R_S), CO₂ transport flux in sap flow (F_T) and storage flux (△S) were calculated according to mass balance method. Our results showed that proportions of E_A, F_T and △S accountting for R_S are dynamic. CO₂ efflux (E_A) on the stem surface closely related with internal CO₂ flux of the stem. It means that F_T and △S can affect E_A. In 24 h cycle, E_A, F_T, △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.

Key words: stem CO₂ efflux, sap flux density, stem respiration, xylem transport, larch