In conjunction with eddy covariance measurements of CO2 fluxes at the 39.5-m height over a 21.5-m-tall boreal aspen stand in northern Saskatchewan, CO2 concentration was measured at eight heights in order to calculate net ecosystem exchange. During both leafless and full-leaf periods, daytime vertical CO2 concentration gradients above 9 m were weak (<0.2 μmol mol-1 m-1), but were strong below this height. Little change in CO2 storage in the air column below 39.5 m occurred during much of the daytime, while around sunrise and sunset CO2 storage changed mainly below 9 m. For the rest of the night, over 85% of the increase in CO2 storage occurred above 9 m. On some calm nights during the growing season, CO2 also accumulated below 9 m resulting in a sudden upward CO2 flux at 39.5 m following the resumption of mixing 2--3 hours after sunrise. A 10-day experiment was conducted to determine the spatial variability of CO2 flux in the trunk space. Two eddy covariance systems were mounted just above the understory about two tree heights apart. The correlation between CO2 fluxes were poor even under unstable (daytime) conditions, suggesting a relatively heterogeneous understory and soil. In contrast, the correlation between water vapor fluxes was high (r2=0.70) in unstable conditions. However, average daytime and nighttime CO2 fluxes over the 10 days agreed to within 5%. This suggests that partitioning net ecosystem exchange between overstory and understory on an hourly basis using a single-understory eddy covariance system is inadvisable; however, partitioning probably can be done quite reliably using 5-day average fluxes. ¿ 1999 American Geophysical Union