extrasense,
"It depends, what kind of tree and what happen to it after it has died."

If it turned into coal during the cretaceous period, most of will be back in the atmoshere by 2050.
And there goes your origional premise.
Fossil fuel that was sequestered, (mostly by forests), over geological ages is being released in a period of centuries.
It is evident that that makes a lot more immediate difference in CO2 levels than a change in forestation.

Pragmatist

Cynic, n: a blackguard whose faulty vision sees things as they are, not as they ought to be. - Ambrose Bierce - 'Devils Dictonary`