Something else you might wanna try - you were saying it's hard to break this kind of thing up. The below link is to an image that was made by creating 5-6 planes of trees (on small mountains), brought into Photoshop for some tweaking, then After Effects for final composite and atmo effects.

http://www.renderosity.com/mod/gallery/index.php?image_id=1449188&member

Since they're broken up, I can even do a slight animation keeping them perpendicular to the camera. Too much and you'll really see that they're flat, but a little movement works just great!

Hope this helps-
-Lew ;-)