So some comments and questions while we are on this topic.

First, what happens to the dryad if their tree is destroyed?
I have always thought of the dryads tree like a lich's Phylactery, so when the tree is destroyed the dryad is no longer within range of it and therefore dies in 4d6 hours.

Second, by RAW or RAI is there any reason liveoak or awaken can't be cast on the dryad's tree?
If it can then it seems to me that the standard dryad would have their tree permanently become a Treant and most dryads would be bound to oak treants rather than just plain oak trees.

Third, if an oak tree is magical enough to form a dryad spirit why isn't it magical enough to form its own spirit and become a treant?