I have read several (over 10) forum threads on how come the trees cant be made into units. For a recent map i am making i needed this also. After some investigation I decided to just try something and BAM I have tree units.
Enclosed is every possible tree combination. Each has its own separate mdl, and while that is a pain you can actually now play birth animations of the trees.
No they do not animate. Im not that good :>
Any questions let me know. Any bugs let me know!
Good luck and have fun.
To get the models just download this map and extract the MDX files.
The file just comes up as a file named Attachment without an extension, I renamed it to Tree.zip, but for other people they might not know to do that, reupload it without the 's