I'm getting a vertical slicer on the textures where the tree isn't rendered. The problem is more evident in animations, where the forest trees are turning sideways then back to facing the camera.
This problem is due to a precision error in the BSP algorithm of Mental Ray. It happens only under certain conditions:
1) Forest is using billboards with Fake shadows enabled.
2) When the Forest items are turning to facing the camera, the Shadow plane becomes almost parallel to the texture plane.
3) The global scale of the scene is much larger than the Forest items size (as example, using large terrains).
This is not specific problem of Forest. The rendering artifacts can be reproduced using a sample mesh, that includes two very closed planes (see image).
There are two possible workarounds for this issue:
1) Change the rendering algotithm from BSP to BSP2 in 'Render Setup->Renderer->Rendering Algorithms'.
2) Enable the 'Scanline' checkbox in the same window.