is there a way to cull the vegetation using xfrog in terragen2 ?
No, you can only use Xfrog to create models and export them as .obj file to import them into TG2. There are many threads regarding this on this forum.
A tuto with an export with XFrog and modification with Poseray.
Sorry this tuto was made in french !
This tuto is here : http://www.christianfly.com/index.php?autocom=Downloads&showfile=274
And if you prefer english here's a brief summary of how I export my models from Xfrog to TG2:
http://forums.planetside.co.uk/index.php?topic=4163.msg43635#msg43635 (http://forums.planetside.co.uk/index.php?topic=4163.msg43635#msg43635)
Good luck and if you have any questions feel free to ask :)
Martin
...and here's my pipeline for xFrog conversions to TG2 - http://forums.planetside.co.uk/index.php?topic=3036.0