Summertime horizontal and vertical advective carbon dioxide fluxes measured in a closed-canopy Douglas-fir forest on a slope

An observational study was conducted during an eight-week period in the summer of 2009 at a forested site in British Columbia (“DF49”) to determine the incidence and magnitude of advective carbon dioxide (CO₂) fluxes. The site is situated in a tall, closed-canopy (zh = 35 m, 1100 stems ha-¹, LAI of 7 m² m-²) Douglas-fir forest on a 5-10o slope. Katabatic flow occurs within the subcanopy during a significant portion of the day, especially during the evening and night. Wind vector and CO₂ concentration measurements were made at heights of 1, 2.6, 8, 20 and 42 m on the main flux tower, while CO₂ concentration measurements were made 73.5 m upslope from the tower at heights of 1, 2.6, 8 and 20 m, creating a two-dimensional transect. Eight thermostated non-differential infrared gas analyzers (IRGAs) were paired using a solenoid valve switching system, creating four differential analyzers. The switching system, combined with a twice-daily polynomial IRGA calibration, enabled measurement of CO₂ concentration differences at the four subcanopy measurement heights to an accuracy of 0.5 μmol mol-¹ (horizontal) and 1 μmol mol-¹ (vertical). The system also recorded semi-continuous CO₂ traces at all sample points. Horizontal advective fluxes, averaged over the main measurement period (August 2 - 25), reached 5 μmol m-² s-¹ at night and 10 μmol m-² s-¹ in the afternoon (a net source to the atmosphere) and -2 μmol m-² s-¹ during the morning (net sink). Vertical advective fluxes averaged around zero during the night, 5 μmol m-² s-¹ during the morning and -5 μmol m-² s-¹ during the afternoon. A cumulative sum of advective flux-corrected net ecosystem exchange (NEE) over the main measurement period (-57 g C m-²) was close to that of the friction velocity-corrected NEE (-60 g C m-²), both predicting less carbon uptake by the Douglas-fir forest than the storage-corrected NEE (-86 g C m-²). Additional CO₂ difference measurements made after the main sampling period along the 73.5-m transect demonstrated that horizontal CO₂ differences increased monotonically down slope (on average) at the 2.6-m height, despite an opposing gradient in manual chamber-measured soil CO₂ effluxes.Science, Faculty ofEarth, Ocean and Atmospheric Sciences, Department ofGraduat