Spatial and temporal variability of CO₂ concentration and flux in a boreal aspen forest

In conjunction with eddy covariance measurements of CO 2 fluxes at the 39.5‐m height over a 21.5‐m‐tall boreal aspen stand in northern Saskatchewan, CO 2 concentration was measured at eight heights in order to calculate net ecosystem exchange. During both leafless and full‐leaf periods, daytime vertical CO 2 concentration gradients above 9 m were weak (<0.2 μmol mol −1 m −1 ), but were strong below this height. Little change in CO 2 storage in the air column below 39.5 m occurred during much of the daytime, while around sunrise and sunset CO 2 storage changed mainly below 9 m. For the rest of the night, over 85% of the increase in CO 2 storage occurred above 9 m. On some calm nights during the growing season, CO 2 also accumulated below 9 m resulting in a sudden upward CO 2 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 CO 2 flux in the trunk space. Two eddy covariance systems were mounted just above the understory about two tree heights apart. The correlation between CO 2 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 ( r 2 = 0.70) in unstable conditions. However, average daytime and nighttime CO 2 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.