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

After harvesting of forests, deforested sites are initially sources of CO2, but generally become sinks for CO2 after some period of years following reforestation. This period for boreal forests has long 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. We sought to determine the timing of the source to sink transition for a sub-boreal clearcut in central British Columbia, Canada. Bowen-ratio measurement of growing season net ecosystem CO2 exchange (NEE) combined with modeled NEE based on ecosystem component CO2-flux measurements for years 5 (1999) and 6 (2000) after harvest showed that this clearcut was still a source for C (NEE of +225 to +263 g C m-2) after 6 years (Pypker and Fredeen, 2002a; 2002b). Measurements of NEE were subsequently made with an open-path Eddy Covariance system in years 8 (2002) and 10 (2004) after harvest and compared with years 5 and 6. Our results suggest that when non-growing season fluxes are considered, this clearcut was carbon neutral in year 8 (NEE of +17 to -21 g C m-2) but a clear sink in year 10 (NEE of -75 to -113 g C m-2). Therefore, our empirical measurements of NEE for a sub-boreal clearcut are consistent with the modeling results that suggest northern conifer forests become net sinks for CO2 at approximately 10 years after harvesting.