Alternative to SpeedTree ?

I've been thinking of the same thing. My conclusion is that you should just write your own. I've seen a few people try to integrate speed tree and they either could not complete the task, or did so and they were just meh anyway.

I think tree rendering is just going to be about having good real time rendering capabilities. You can just load artist created trees, it'll be most important how you render them. You'll have to come up with all kinds of imposter and offline caching of information like lighting in order to render realistic tree scenes.

If i'm not mistaken, that task is not handled for you by speed tree. You still have to do all the rendering management yourself. So just start there.

edit: I was looking more closely at speed tree and it was looking pretty sweet. the 360degree normal mapped billboards especially. Unforunately neither of us can afford that and so we must write our own :P