Barkskin

2nd-Level Transmutation
Casting Time: 1 action
Range: Touch
Components: VSM (a handful of oak bark)
Duration: 8 hours

You touch a willing creature. Until the spell ends, the target's skin has a rough, bark-like appearance. The target gains the following features:

Armor Class. The target's AC is 17, and it can't be reduced below this number. It can benefit from a shield with this armor class, but not its Dexterity modifier.
Treelike Resistance. The target is resistant to bludgeoning and piercing damage.
Kindling. The target is vulnerable to fire damage.

The target can use a bonus action or a reaction to end this spell early.

At Higher Levels. When you cast this spell using a spell slot of 4th level or higher, the target's AC increases by 1 for every 2 slot levels above 2nd.

Source: PHB 217, SRD

Changelog

Added upcasting option.
Removed concentration requirement.
Reworked AC calculation, Added extra resistances & vulnerabilities.
Increased base AC to 17, duration to 8 hours, and the ability to end the spell as a reaction.