In spite of the importance of respiration in forest carbon budgets, the mechanisms by which physiological factors control stem respiration is unclear and estimates of respiratory emissions from stems and branches are quite variable. Internal CO₂ concentration ([CO₂]) and temperature are important factors influencing the rate of CO₂ efflux from stems. Stem CO₂ efflux can originate from respiration of local cells in the inner bark, cambium and xylem rays but can also come from root respiration and transport of CO₂ in xylem sap. However, sap flow is often negatively correlated with [CO₂] and stem CO₂ efflux. Variability in the rate of CO₂ efflux from stems and branches can also result from corticular photosynthesis. The objective of this experiment was to improve our understanding of the factors controlling CO₂ release from stems of Eucalyptus trees; namely: (1) the relationship between stem CO₂ efflux, xylem [CO₂], the rate of sap flow, and temperature; and (2) the importance of CO₂ re-fixation in the stem. CO₂ efflux was measured in stems of five-and ten-year-old Eucalyptus globulus trees located in two stands in central Portugal. Xylem [CO₂], stem temperature and sap flow were also measured. Measurements were repeated three times during the day under lighted and shaded conditions.

Results/Conclusions Results showed that both temperature and xylem [CO₂] are important factors influencing the rate of CO₂ efflux from stems. In spite of differences observed between trees of different age, the fitted regression lines had similar slopes suggesting similar relationships between the variables. No relationship was found between CO₂ efflux and sap flow rate, indicating the presence of strong barriers to diffusion of CO₂ from stem to atmosphere. Under lighted conditions, refixation of CO₂ by corticular photosynthesis was only 7% of CO₂ efflux in shaded conditions. Two possible reasons for the low rate of CO₂ re-fixation could be low incident irradiance and/or photoinhibition resulting from high internal [CO₂].