Hi,
Thanks for all provided assets. Thought all objects in the main scene "MissingTreeTest.max" seem to correct, we've found, that the "Forest General" Forest object in the "Ash-Pine-Maple.max" is corrupted (unfortunately there's no data identifying what happened). I would suggest you to recreate such object, that should avoid these problems.
Actually, a recommendable work-flow would be to link these XRefs directly to source objects (stored in ash_tree_03.max / maple_1_03.max).
Best regards,