One analysis I read some time back claimed that Trees are net positive when they are closer to equator, IIRC 30deg each way. fortunately for this topic, the areas close to equator are also significantly poorer countries thereby maximizing the roi of a fixed dollar expense.

Overall I believe that if a functioning carbon credit marketplace is established we can fund the whole green movement with a fraction of world GDP and might even end up creating a whole bunch of jobs doing it.