Most of the tree models used by the alpha testers, including myself and Luc, are
Xfrog Plants from Greenworks. They offer a number of libraries of high quality plant models at several stages of development (young, adult, mature). They cover trees, bushes and flowers, house plants, exotics, cacti, even underwater species. They're not cheap, but generally speaking the quality is worth it.
You can find a sample of TGO format Xfrog models for free download on our TG2 Technology Preview page here (bottom-right):
http://planetside.co.uk/terragen/tg2/tech_preview.shtml TGO versions of the full Xfrog plants library are not available at this time but I encourage you to inquire about the possibility.
- Oshyan