Spell level : Innate level: 2, Druid: 2, Ranger: 2, Other: Cleric with Plant domain 2
Components : Verbal and Somatic
Range : Touch
Target/Area : Single
Duration : 1 Hour * cLevel
20 Rounds (cLevel).
Save : Harmless
Spell resistance : No
Barkskin hardens the target creature's skin, granting a natural armor bonus to armor class based on the caster's level. The spell grants four armor class with one more armor class for every three caster levels. You gain another 3 to natural armor class for each Epic Relevel attained. Druids, Rangers and Spirit Shamans on the other hand, gain 4 per Epic Relevel.
Note: The Content Library on the RoT website gives a different description.
Barkskin hardens the target creature's skin, granting a natural armor bonus of 6 + your levels of caster divided by 4 to Armor Class. The maximum AC bonus of this spell is capped at +15 for Clerics with Plant domain.
It's not known at this time which is correct.