Spatial and temporal variability of CO2 concentration and flux in a borealaspen forest

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, CO 2 concentration was measured at eight heights in order to calculate net eco system exchange. During both leafless and full-leaf periods daytime vertica l CO2 concentration gradients above 9 m were weak (< 0.2 mu mol 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 th e night, over 85% of the increase in CO2 storage occurred above 9 m. On som e calm nights during the growing season, CO2 also accumulated below 9 m res ulting in a sudden upward CO2 flux at 39.5 m following the resumption of mi xing 2-3 hours after sunrise. A 10-day experiment was conducted to determin e the spatial variability of CO2 flux in the trunk space. Two eddy covarian ce systems were mounted just above the understory about two tree heights ap art. The correlation between CO2 fluxes were poor even under unstable (dayt ime) 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 CO2 flu xes over the 10 days agreed to within 5%. This suggests that partitioning n et ecosystem exchange between overstory and understory on an hourly basis u sing a single-understory eddy covariance system is inadvisable; however, pa rtitioning probably can be done quite reliably using 5-day average fluxes.