Barkskin doesn't stack with the natural armor gained from wildshaping into an animal. Ie a bear has 15 dex and +3 natural ac. So you wildshape and have 15 ac. Barkskin won't help wildshape unless its giving more than that.
Barkskin
Transmutation
Level: Drd 2, Rgr 2, Plant 2
Components: V, S, DF
Casting Time: 1 standard action
Range: Touch
Target: Living creature touched
Duration: 10 min./level
Saving Throw: None
Spell Resistance: Yes (harmless)
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.
... a druid with a wild armor, animated shield, ring of protection, barkskin ... can get a really good AC in wildshape now 
