When do replanted sub-boreal clearcuts become net sinks for CO2?

After forest harvesting, sites are initially sources of CO2, but eventually become sinks for CO2 after some period of years following reforestation. This period for boreal forests has been assumed to be 10 years, but this has not been validated empirically for most forest types including sub-boreal spruce-dominated forests of central British Columbia, Canada. Therefore, we sought to determine the timing of the source to sink transition for a sub-boreal clearcut. Clearcuts such as the one documented in this study occurring on glaciolacustrine deposits with relatively poor drainage represent about 20% of the 1.5 million ha in the Prince George area. Net ecosystem CO2 exchange (NEE) for a clearcut was measured over four growing seasons in years 5, 6, 8 and 10 after harvest. A Bowen ratio approach in combination with a bottom-up modeled NEE based on ecosystem component CO2-flux measurements was used for years 5 and 6. In years 8 and 10, growing season NEE was measured using an open-path eddy covariance system. A cross comparison of Bowen ratio and eddy covariance systems was performed and measurements agreed relatively well (r2 = 0.58). The results demonstrated that while this clearcut was still a source for C (NEE of +336 to +487 g C m−2) after 6 years, it was most likely a sink for C between 8 (NEE of −189 to −52 g C m−2) and 10 (NEE of −185 to −48 g C m−2) years following harvest.