Hi,
I would suggest you to double check the UV mapping of used Surface and if the Map channel is correctly set under Transform - Scale - Map - Map channel (by default set to 1).
Let me add, that marking the Surface - Auto check-box (to Update surface data automatically) would be handy to update your Forest object(s) on any surface change (if not, it'll update while building data for rendering).
Attaching a sample scene & screen-shots of its set-up. Hope you'll find it useful.
Best regards,