A222: It's totally DM dependant, and unsuitable for PC's because of the extreme limitation. Your DM will not let you play a Dryad, since he want's to do other things, rather than stay in a 200 yard area of your tree. If you're the DM, who cares?
Seriously though, I would say a +2, if you can get a way to carry the tree around with you. Mostly because of the entangle at-will SLA. Ability Modifiers would probably be +2 Dex, +2 Int, +2 Wis, +4 Cha. Maybe have the oak planted into the deck of a ship, then have it act as a mast.