I don't think this is the complete picture. In an intact old forest, a lot of bio-matter doesn't decompose in time, but rather gets buried in the anaerobic zone. In a healthy forest the first few centimeter soil are so biologically active just about 30cm down there is very little oxygen left to house much life. That's also why murder victims don't decompose very fast, when the murderer buried them deep in forest soil.
Granted this still depends on the forest's life, but it is beyond an individual tree's lifecycle.
I think it's pretty easy to see, though, that restoring the world's forests to their condition circa 1700 will not get us to 1700 CO2 levels, as we have added some 1.5T tons of fossil carbon to the mix since then.
Granted this still depends on the forest's life, but it is beyond an individual tree's lifecycle.