You don't need to buy land specifically. In fact, that's probably counter productive. A better bet would be to seek a lease or an easement in someone's plot for a fee.
if you don't own the land, you can't stop the owner from chopping down all the trees you planted and letting the CO2 exhaust into the air again. i guess you could try putting it into a contract, but it would de facto be unenforceable because you wouldn't know if the land owner was cutting down trees on their property anyway.
that's why you purchase an easement, and yes, it would be enforceable. I mean, it would be as enforceable as making sure no one poached your trees on your vast swaths of remote, distributed acreage. In my scheme, because the revenue is recurring, the owners would have every incentive to prevent poaching, thus potentially relieving you of some extra costs.
I suppose a tree hating owner could sign up and then chop down your trees, but luckily, you can probably sue him for breaking a contract, or just not let him have another tree.
