The CO2 efflux from tree stem surfaces to atmosphere (RS) is an important component in the carbon (C) balance of forest ecosystems. Rising atmospheric carbon dioxide concentrations [CO2] are expected to stimulate RS, because of greater C assimilation and carbohydrate supply to stems under rising [CO2]. Growth respiration (Rg) and maintenance respiration (Rm) during the warm growing season may respond differently to rising [CO2] due to different metabolic demands. To test the effect of elevated [CO2] (eCO2, ambient +150 ppm) on RS, we examined RS in mature Eucalyptus trees on a monthly basis for an entire year during the seventh year of exposure to eCO2. RS varied seasonally and mirrored seasonal variation in temperature. While RS was not significantly increased under eCO2 compared to ambient CO2 (aCO2), its temperature sensitivity was significantly decreased (Q10 of 1.92 for aCO2 and 1.56 for eCO2). The estimated annual Rg accounted for approximately 7–8% of annual total RS, 419 ± 103 g C m−2 yr-1, indicating that Rm contributes substantially to total RS in this mature woodland. Monthly mean RS was correlated with monthly mean soil temperature, soil moisture and monthly stem growth rate in this dry year, but soil moisture levels may have been insufficient to observe the impacts of eCO2 on stem growth in this droughted and phosphorous limited site. Our results highlight that eCO2 tends to increase Rm at low temperatures during the non-growing season, thus decreasing the temperature sensitivity of RS, despite a neutral effect of eCO2 on RS rates on a yearly basis.