Forest Pack is the only piece of software I can think of, which enables you to create such organic, versatile structures, in such an easy and flexible workflow.
My goal was to try to design a scene full of subtle details. I had no clear concept when I started, just a blurry image in my mind. So the scene developed and grew bigger and bigger while working on it. A great help for me regarding development and improvement is always the direct communication with colleagues, friends and community-members starting in a quite early w.i.p.-stage.