SRD:Barkskin

Barkskin toughens a creature’s skin. The effect grants a +2 enhancement bonus to the creature’s existing natural armor bonus. This enhancement bonus increases by 1 for every three caster levels above 3rd, to a maximum of +5 at caster level 12th.

The enhancement bonus provided by barkskin stacks with the target’s natural armor bonus, but not with other enhancement bonuses to natural armor. A creature without natural armor has an effective natural armor bonus of +0.