or take the Hollywood approach- just render the trees at the edge of the forest, maybe a row in from that (depends on your angle), and put something dark and forest-matlike behind that, obscured for the most part by the trees. You can get a vastness also by bringing the camera down almost to the level of the plane, then pointing slightly upwards (and I do mean slightly, unless they're birdmen..;)
one thing I've done is to create a terrain, lop off the really distinctive features, then duplicate and rotate each copy to a random orientation on the 'y' axis. Just my 2p..;)

I wish I'd said that.. The Staircase Wit

anahl nathrak uth vas betude doth yel dyenvey..;)