Hi,
Forest object(s) can use XRef object(s) as assigned surface(s) and build that way it can be again referenced (placed as a XRef object) in your main scene. This way it can be divided into three scenes (surface(s), Forest object(s) on XReffed surface(s), XReffed Forest object(s)) and it'll always update on any change in those.
Hope that helps.
Best regards,