It looks great as is, but it looks like a small bush. trees would tend to have separate brances, thus separate bushes. think of the tree as a pile of bushes. just like on tsugumo's tutorial, where he talks about that tree from world of mana 3, or sword of mana. dont remember now.
the tree was made of several spheres, you know? and that would influence on the shading, making it not so smooth as yours.
btw, the style of shading you got there is really nice!! go go go!!!
Kaz