Is not possible to use Forest in a XRef scene because the trees don't update with the camera. You can load the terrain as a XRef object and link Forest to it, but i believe that won't be very useful because the memory used will be the same that using the terrain as a mesh.
What about if you create a simplified terrain (applying an 'Optimize' modifier or deleting the unused triangles) only for Forest ? You would set it to non-renderable to hide from the render. Also, i think that will be more easy to place the trees on a single surface, that using several terrain sections.
Don't forget to convert the optimized terrain to an editable mesh, or Max won't free the memory used by the deleted triangles.