Carbon sequestration in British Columbian sub-boreal cut blocks

During the summers of 1999 and 2000 measurments of component and ecosystem-level C02 fluxes were taken in seven cut blocks aged 0, 2, 3, 5, 6, 9 and 10years since harvest in the Aleza Lake Research Forest, near Prince George, British Columbia. A Bowen ratio energy balance (BREB) system was placed within a 5 to 6 year-old planted interior spruce (Picea glauca x Picea engelmanniz) clearcut to measure the ecosystem C02 flux during the 1999 and 2000 growing seasons. To substantiate the BREB measures of the ecosystem C02 flux a second method, called the component model, was established based on instantaneous measurements of component C02 fluxes (conifer, herbaceous plant, woody shrub and below ground) scaled up to the ecosystem-level. Both approaches indicated inter-annual variation in the growing season C02 fluxes. The 1999 growing season ecosystem C02 flux measurements estimated the clearcut to be a sink of -20 ~ 43 g C m~2 and -86 g C m~2 from 27 June to 3 September, using the BREB method and component model respectively. In contrast, in 2000 the growing season C02 flux during the same time period was a source of 43 g C m~2 (BREB method) and 66 ~44 g C m~2 (component model) during the same period. In 2000, an additional 50 days of measurement, from 24 May to 20 September, indicated an even larger source of C02 totaling 143 ~57 and 103 g C m~2 using the BREB method and component model, respectively. However, regardless of the approach used, both years would have been sources of C02 if additional measures were taken for the entire year. The component fluxes were dominated by the C02 source from below ground (338 and 466 g C m~2 in 1999 and 2000, respectively) and the C02 sink provided by the deciduous plants (-382 and -365 g C m~2 in 1999 and 2000 respectively). The conifer photosynthetic C02 uptake for the 5 to 6 year-old clearcut was meager in comparison (-47 and -57 g C m~2 in 1999 and 2000, respectively). During the 2000 growing season, the below ground C02 flux and above ground biomass was measured in 7 cut blocks aged 0, 2, 3, 5, 6, 9 and 10 years and one mature stand. All sites were in the SBS wkl 08 or 07 except the 0 and 9 year-old sites that were of the sub series 01. The growing season below ground C02 flux was similar for all cut blocks, including the mature forest. Furthermore, there was little change in above ground biomass carbon uptake between the different aged cut blocks. Hence, when biomass was incorporated with the below ground C02 fluxes all cut blocks were sources for C02\u2022 Therefore, cut blocks within the Aleza Lake region (SBS wk1 07 or 08) are sources of C02 for at least 10 years after harvest because the conifer component of the C02 flux is incapable of surmounting the loss of C02 from below ground.