Attached Link: http://www.renderosity.com/mod/gallery/index.php?image_id=266155

I dont know if its really going to be a problem? I've made a forrest with thousands and thousands of bryce native trees on a pc with only 512mb of ram. It rendered really fast and without any problems whatsoever! Just lose the soft-shadows, cuz they'll make ur rendering life a living hell haha.

For real far distant trees you could also use a terrain with a noise to the displacement and some matching noisy DTE texture with colors much similar to the colors of your trees. This can be used to create the illusion of far mountains and hills overgrown with loads of trees.
You can see the result of my attempts at:
http://www.renderosity.com/mod/gallery/index.php?image_id=266155

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