Here's part of an assignment from the latest cartoon college blogHere are a bunch of different styles and types of trees. They all have things in common: Good design and hierarchy, and good composition within the BGs.
The overall shape of each tree is interesting and then the sub shapes fit nicely within the big shapes - wrapping wround them. Then the smallest details do the same.http://johnkstuff.blogspot.com/2008/05/more-bg-layout-note-hierarchy-of-form.html
http://johnkstuff.blogspot.com/2008/06/bg-layout-tips-for-nate.html
http://johnkstuff.blogspot.com/2008/06/howie-post-tree-king.html