You could do collision column model as object in xpack with transparent texture and place around the tree trunk. Eno72s idea.
Also try put really low LOD to it like 1m or something, then RBR will render it just before you going to hit a tree. This way you probably save FPS. Never tried myself yet though
this actually is a workaround. I use this for three reasons:
1 - the btb collision column does not seem to work: collision with whole tree whatever the dimension of the collision column in xpacker (?)
2 - on a series of identical objects, if you set only some of them collideable, in RBR there is a big chance they won't be rendered
3 - it's a nice way to simulate "soft" collisions: for instance I used this for the tyres on the ground, making a flat and smooth invisible mesh, and putting it inside the tyre. The result is a small bump of the car and not a complete wrecking.
to make it work best, edit the xml files of the xpack so that the collision is already "TRUE". Otherwise, when adding them in btb on top of other objects you will have to select them to set them collideable, but it won't be easy. Also set already in xpack a very low LOD.
The reason for using a transparent texture (even 2x2 pixels is ok) is that these "collision meshes" must be actually rendered, otherwise you won't collide with them.
Hopefully the collision column will be fixed in btb, in the meantime I believe these can be good tips.