Forest harvesting and the carbon debt in boreal east-central Canada

Conversion of carbon-rich, primary boreal landscapes to managed ones through clearcut-based silviculture has the potential to decrease landscape-level carbon storage and thereby incur a significant carbon debt. We calculated carbon debts and payback periods associated with production of wood pellets to replace coal, oil and natural gas in electricity generation for such landscape conversion in boreal east-central Canada. Local forest inventory information in combination with the Carbon Budget Model (CBM-CFS3) was used to estimate biomass and dead wood carbon stocks after fire or clearcutting, and resulting age- and disturbance-specific carbon stock estimates were used to populate simulated landscapes. Based on empirical information, we investigated a range of fire-return intervals in the primary landscapes (114–262 years), harvest rotation ages (80–100 years) and conversion efficiency factors (0.17–0.71 tonnes fossil fuel carbon eliminated per tonne harvested wood carbon). After a first rotation of harvesting, carbon stocks declined 33–50% relative to stocks in the natural, fire-dominated landscapes and payback periods ranged from 92 to 757 years. The type of fossil fuel had the strongest effect on payback periods: under average efficiencies, ranges were 122–207, 156–268 and 278–481 years for coal, oil and natural gas respectively. These calculations suggest that under a wide range of assumptions, clearcut-based management of boreal primary landscapes to produce wood pellets to replace fossil fuels in electricity generation will result in net emissions of greenhouse gases to the atmosphere for many decades.