As far as a map like that would go, the trees would be consider a frame. I almost always start with a very rough idea of were the frame is going to lay out (usually just wish a border line). I work on the actual frame and its detail very last. I try to do digital work like I do real painting, which you pretty much always start in the background and sky color and work to the foreground. No real wrong way to do it necessarily (especially digitally)