Oh, North Canada? That's some tricky terrain. Bedrock is very near the surface, and you're right, it's hard to build up. Short growing season, for one.

The article you link mentions softwood forests being problematic for carbon accumulation. Not hardwood forests. Old growth temperate hardwood forests are quite rare now, and some of the apex tree species in those forests have very serious pathogens keeping them from re-establishing, more's the pity.