Hi,
Any overrides of the "auto-material" assigned to the Forest object aren't recommendable, because Forest keeps references to its source materials. Instead, you should modify the material linked to that item - 
Custom Object(s) - 
Material - any changes made this way, will be projected safely to the Forest's "auto material". Also, if preferable, without having 
any material assigned under Forest's 
Geometry > 
Material, the Forest object is getting the material directly from the Custom Object(s). I hope that clears it up.
Best regards,