Effects of Logging on Carbon Dynamics of a Jack Pine Forest Chronosequence in Saskatchewan, Canada

The boreal forest covers ~10% of the world's land surface area and contains a disproportionately high amount of global terrestrial carbon (C). However, it is uncertain whether Canada's boreal forest is currently an annual net C sink or source and what role disturbance plays in determining this. The goal of this study was to calculate C budgets for a chronosequence of harvested jack pine (Pinus banksiana Lamb.) sites (1-y-old, 5-7-y-old, 10-12-y-old, ~29-y-old) and a ~79-y-old site established by wildfire. We measured total ecosystem C content (TEC) and above- and belowground net primary productivity (NPP) for each site. Total annual NPP (Mg C ha^-1 y^-1 +/- 1 S.D.) from May 1999-April 2000 was 0.9 +/- 0.3 at the 1-y-old site, 1.3 +/- 0.1 at the 5-7-y-old site, 2.7 +/- 0.6 at the 10-12-y-old site, 3.5 +/- 0.3 at the 29-y-old site, and 1.7 +/- 0.4 at the 79-y-old site. We measured soil surface CO₂ fluxes (R_S) using an IRGA with chamber and correlated these measurements with soil temperature to estimate annual R_S. To calculate net ecosystem productivity (NEP), we estimated heterotrophic respiration (R_H) as 50% of total annual R_S, and summed NPP and R_H. NEP (Mg C ha^-1 y^-1 +/- 1 S.D.) from May 1999 through April 2000 was estimated to be -1.4 +/- 0.3 at the 1-y-old site, 0.1 +/- 0.1 at the 5-7-y-old site, 1.1 +/- 0.7 at the 10-12-y-old site, 0.6 +/- 0.4 at the 29-y-old site, and 0.1 +/- 0.5 at the 79-y-old site. The 1-y-old clear-cut was a source of C to the atmosphere; all other sites were either sinks or were essentially C neutral. We also calculated long-term annual NEP values based on the difference in TEC among the five differently-aged sites. These figures agreed well with the flux-based NEP estimates, except in the case of the 1-y-old site, which we determined had been a more productive site with greater forest floor carbon prior to harvest.