Nope, as of right now, only static models are supported. Forget having them in a forest item. Plus if the trees in the forest were jbeamed that would quickly become too complex for most computers.
I want to create jbeamed grass (and maybe also trees and some crops in the future). I don´t want it as a "vehicle", just a forest item with jbeam instead of static collision.
Ah yes, can't do that and won't be able to do that. Crushing/bending grass is normally done with "tricks" in other games btw - physics is not involved.