This is a known problem in the recent releases of V-Ray, but i'm not sure if it happened also in previous versions (V-Ray 2 and before). This may be the cause that HQ Plants uses Normal Bump instead VrayNormalMap in their materials.
We'll inform to 3D Mentor
about the problem, so they update their libraries. Unfortunately we cannot make changes on third-party products.
Thanks for you suggestion about Material Optimizer. We'll add this functionality in next updates.
I could reproduce the error rendering locally, removing Forest Color and selecting a frame range. Rendering a single fine works fine.
But the behaviour would be different in your render farm. If the problem is not fixed using VrayNormalMap, please tell us (in case there is a different problem).
Anyway, elements which don't use Forest Color (as trunks) are rendered directly by V-Ray. Forest should not affect in any way, because doesn't participate in the render process.
Apologies by the incoveniences.