The following rules apply to all creatures with the Forest Spirit special rule.
Non-Forest Spirits may not join a Forest Spirit unit and vice versa. Treeman Ancients may never join units.
Forest Spirits have a 5+ Ward Save. However, this save cannot be used against magical attacks (spells, magic weapons etc).
All attacks by Forest Spirits count as being magical. This includes any special or ranged attacks they may have.
Forest Spirits are Immune to Psychology.