My guess would be that a fair bit of the CO2 will be released into the atmosphere but probably not all of it. An interesting thing to consider though is that trees do reproduce on their own if given an appropriate environment. If a forest is created where there was no forest before then the CO2 associated with those trees will be pulled out of the atmosphere. Eventually the forest region will probably reach an equilibrium with the environment: absorbing CO2 to produce new trees while at the same time emitting CO2 from decomposing trees. The change from "no forest" to "forest" will be a net decrease in atmospheric CO2.