Bettula platyphylla Suck., Larix gmelinni (Rupl.) Rupl. and Flaximus mandshurica Rupr. are key tree species in the secondary forests in Northeast China, which play an important role in the carbon balance of temperate and boreal forest ecosystems. In temperate climates with four distinct seasons, large differences in temperature and sap flow velocity between spring and autumn may result in a significant difference in stem respiration. In the current study, we tested the effects of temperature and sap flow velocity on the CO₂ efflux from stems of the birch, larch and ash trees in both June and September of 2005-2007 by using the Li-7000 infra-red CO₂/H₂O gas analyzer (IRGA) in situ. Measurements were carried out at two locations; one in a 35 year-old temperate larch plantation mixed with ash and birch in Laoshan Forest Experiment Station in Northeast of China (E127°34',N45°20') and the other in the Botanical Garden of Northeast Forestry University (E126°38',N45°43') with plenty of planted birch, larch and ash trees (ca. 20 years old). Strong diel and seasonal variations in respiration rates and sap flow velocity were found in all the three species tested. Diel dynamics of CO₂ efflux and sap flow velocity showed various unimodal-curves depending on the season. The peak of CO₂ efflux in June occurred during the scotophase which is contrary to that of the temperature, whereas both the CO₂ efflux peak and the maximum temperature co-occurred at the mid-day in September. Stem CO₂ efflux was much higher in June than in September. Daily mean stem CO₂ effluxes (September/June) of the birch, larch and ash were 0.82/3.32, 0.74/3.78 and 1.98/4.98 μmol · m^-2 · s^-1 respectively; daily mean sap flow velocities (September/June) were 2.48/10.02, 4.78/10.71 and 2.69/7.93 g · cm^-2 · h^-1, respectively. Significant correlations were found among stem temperature, sap flow velocity and stem CO₂ efflux for the three tree species. There were significant positive correlations between the temperature and the stem CO₂ efflux for both seasons; negative correlations between stem CO₂ efflux and sap flow velocity for June; and positive correlations in September for all three species. Effects of the sap flow velocity from stems of birch and larch on their CO₂ efflux were different between the sap flow upward and downward phases (morning and afternoon). This might be due to significant temperature differences in the 24 hour-cycle that cause the variations in CO₂ solubility within the stems, which further alters the balance between internal CO₂ concentration and surface CO₂ efflux. Such balance changes varied by tree species. This might indicate an interactive effect between sap flow velocity and temperature on stem CO₂ efflux. The difference in CO₂ efflux from stems of ash trees between June and September was mainly influenced by the sap flow velocity, but as to the birch and ash, the seasonal difference might result from variations of both temperature and sap flow velocity. Our results showed that temperature is a key factor influencing stem respiration rate; however, the stem respiration (release of CO₂ into the atmosphere) is also affected by the sap flow velocity. The effects of temperature and sap flow velocity on CO₂ efflux from stems of the birch, larch and ash differ between the spring and autumn.