Hi, David. Fancy meeting you here. I don't mean to discourage or disappoint, since you've invested money in Carrara already, but I think for what you want to do, your best course of action is to download and install the FREE addon for Blender called Sapling, which produces some very realistic trees, and use it along with the instancing system in Blender. With this combination, you can easily customize your own trees with tons of variables, and generate a vast forest scene, directly in Blender.

https://www.youtube.com/watch?v=xmQlcAcLOaU

I have only used Carrara some years ago briefly, for a collaboration project, but I know the render engine is ok (nowhere in the league of some of the best biased engines, like Vray). The shader system is powerful, but showing it's age. I really hate to say it, but unless you want Poser or Daz compatibility, the functionality you purchased the software for could have been had for free directly in Blender, with some simple addons. Not knocking Carrara here, so please no flames.

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Tools :  3dsmax 2015, Daz Studio 4.6, PoserPro 2012, Blender v2.74

System: Pentium QuadCore i7, under Win 8, GeForce GTX 780 / 2GB GPU.