It's not terribly difficult to do a combination these days.
If you have a blue sky, it's relatively simple to key it out like one would a blue screen. Then you can drop in mountains, different coloured sky, "massive, angular chunks of an alien planet, floating in space. A dying, shattered world."
All the while, the foreground elements of trees and cast have a realistic look about them and you don't have create digital or model trees or anything.