Hi,

I just remembered something which compliments garyandcatherine idea.

"2.  After you use the populate command, use your erasure brush and delete the plants that are over the horizon and those that are out of the boundary lines of the camera---if you cant see them, why plant them?"

I use cubes/shapes and stretch to cover the area you don't wont trees, below the ground as well, in your case the back of the mountains and populate with the eco system.

You have to leave/turn on 'Decay near foreign objects'  Under the density tab, then use the paint facility to tidy up if needed.
 
Then make the cubes invisible/transparent with shadow casting off etc. 

Hope that helps.

Chris

Edit:

Don't forget you can create your own billboards as well.