I haven't studied the subject to any depth, but I think we are talking two different things here. The peat that covers most of Canadian wilderness, including the arctic tundra which won't support tree growth, is a living breathing thing, and as such probably contributes at least as much to the carbon recyling efforts of mother nature as tree growth does. However it is only the actively growing surface of this peat (moss) that performs the beneficial actions.

Peat bogs, as described here, can be 10's of meters thick or more. Although they are the end product of thousands of years of peat growth, other than the immediate surface growth, they contribute nothing towards nature's cleansing actions any more than the oil and gas deposits found at depth do.

Best regards

K