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The growing season carbon balance of a sub-boreal clearcut 5 years after harvesting using two independent approaches to measure ecosystem CO2 flux

From 27 June to 3 September 1999, CO2 fluxes from a 5-year-old, 84.15-ha vegetated clearcut in sub-boreal British Columbia were measured using a Bowen ratio energy balance (BREB) system and a second approach (the com- ponent model) that was based on scaled up CO2-flux measurements from belowground and plants (spruce seedlings and representative deciduous species). Over the 69-day study period both methods estimated the site to be a small sink for CO2 (-22.4 and -85 g C·m -2 , respectively). Differences between the sink size of the two approaches largely resulted from a divergence in the data after 7 August when the BREB data indicated a switch from sink to source approxi- mately 14 days in advance of the same change from sink to source seen in the component model data. The main com- ponents of the CO2 flux within the clearcut were belowground respiration (338 g C·m -2 ) and deciduous plant photosynthesis (-375 g C·m -2 ). The conifer seedlings were only a minor component in overall CO2 flux over the grow- ing season (-48 g C·m -2 ). The small overall sink estimated for the site for the approximately 2.5-month growing period would likely have been surmounted by the belowground respiration if the yearly CO2 fluxes had been taken into ac- count. For example, an additional 68 g C·m -2 was added to the atmosphere from 3 to 23 September (based on belowground respiration data only), after deciduous plants senesced. This source alone was enough to push the site from a sink to a source for CO2.