Before you go much farther on that, I think it would benefit you a bunch to look really closely at pics of trees as you're spriting. Try blocking in colors and making an appealing shape, thinking of how the branches will move and grow and how the leaves will cluster. Even cartoony trees tend to begin that way, the exaggeration coming after the basic foundation is laid down - perfectly spherical is usually fairly unappealing to look at, even moreso with a fancy leaf texture overlaid on top imo.

edit: here is a prety good example of what I'm talking about, sorry if I'm just babbling