Well, when the problem is that the trees intersect when moving, the solution is... don't move them
With a very high density of trees, you may disable the 'facing camera' option because there are so much vegetation in the screen that you don't notice the effect of planar trees. Please, try this:
- Use a custom very dense bitmap (the dense.bmp included with Forest may be don't enough for this job)
- Disable 'trees facing the camera'
- Apply a random translation (about 30%), random rotation (0 to 360) and random scale
- A random tint may be useful too
- Use a dark texture for the ground, so you can 'hide' more easily the spaces between trees
You will get a very dense Forest, and the camera motion won't be a problem anymore for the animations. I've attached a sample scene, verydense distribution bitmap and render image to show it.