By not giving us exact figures, I find you're allowing yourself a bit of poetic license where this issue is concerned.
What's unique about the boreal forest is that growth occurs very slowly. Trees already store a certain amount to carbon. New carbon emitted by the oil sands extraction process is absorbed by new tree growth, which only occurs three months of the year. If a tree grows “x” number of centimetres, it will absorb “x” amount of carbon, compared to a 20-foot high tree. Consequently, a boreal forest is almost incapable of absorbing new carbon emissions.