When objects are inside of a XRef, is not possible to assign references to elements belonging to main scene, this is a general limitation of XRefs. If you need to link cameras or lights to a object forest, should do a merge of the XRef record, however this could be a problem if you need to merge also the inclusion splines or terrains. Another solution is to use a scene settings where Forest doesn't reference any light or camera.
If you could send us a reduced set of your xref scenes (the minimun objects for see the problem), we could study it and suggest you the best way to include Forest in your workflow.