Key messageEfflux-based estimates of stem respiration in oak trees during the dormant season were biased by axial diffusion of locally respired CO2. Light-induced axial CO2diffusion along the stem due to woody tissue photosynthesis may lead to equivocal estimates of stem respiratory coefficients during the dormant season, which are generally used to estimate maintenance respiration throughout the year.ContextStem CO2 efflux (EA) does not reflect respiratory rates of underlying tissues. Recent research has focused on the significance of CO2 transport via the transpiration stream. However, no studies have yet addressed the potential role of light-induced axial CO2 diffusion on EA during the dormant season when there is no transpiration.AimsThis study investigated to which extent woody tissue photosynthesis and axial diffusion of respired CO2 affect EA during the dormant season.MethodsEA was measured in a stem cuvette on dormant oak trees in a growth chamber at constant temperature. Different rates of axial CO2 diffusion were induced by woody tissue photosynthesis by means of illuminating stem sections at varying distances from the stem cuvette, while light was excluded from the remainder of the tree.ResultsAxial diffusion of respired CO2 led to reductions in EA of up to 22% when the stem section closest to the cuvette was exposed to light.ConclusionDormant-season efflux-based estimates of stem respiration might be biased by axial diffusion of respired CO2, particularly in open forest stands with sufficient light penetration. Consequently, this may lead to ambiguous estimates of dormant season EA coefficients (Q10 and EA_0) generally used to estimate maintenance respiration throughout the year.