Stem CO2 efflux is a highly variable component of the carbon budget of forest ecosystems. It reflects the balance between the CO2 respired by the living stem tissues, less the CO2 dissolved in the xylem sap moving upward in the transpiration stream, plus the CO2 transported from the roots and released at the stem. Although knowledge about such fluxes at different spatial and temporal scales has markedly increased, knowledge of the effects of silviculture treatments, such as fertilization, on stem CO2 effluxes are still limited, particularly when connecting above- and belowground processes. Using measurements obtained from twin plots (one control, one fertilized) installed in five-, 12- and 23-year-old stands of Pinus radiata, the objective of this study was to examine the influence of the temperature, stand age, fertilization, season and time of measurement (day/night) on the stem CO2 efflux, soil CO2 efflux and their ratio. There was a strong significant positive relationship between the stem CO2 efflux and temperature. The slope between these two variables declined as the stand age increased and was higher for nighttime than daytime measurements. The stem CO2 efflux was higher in the fertilized plots compared with the unfertilized (control) plots for the 5- and 12-but not the 23-year-old age classes. In contrast, the soil CO2 efflux was largely unaffected by the temperature, time of measurement (day/night), fertilization and stand age; however, significantly higher values of soil CO2 efflux were measured during spring than during the other seasons. Given the relative invariance of the soil CO2 efflux to the temperature and treatment effects, the ratio of the stem: soil CO2 effluxes was affected by the same factors as the stem CO2 effluxes. These results suggest that fertilization would increase wood production and wood CO2 efflux without changing the soil CO2 efflux, thus most likely proportionally increasing aboveground C partitioning and decrease belowground C partitioning, with this effect being enhanced at younger ages.