You can take on the sturdiness of the mighty oak.
Prerequisite: Ability to wild shape into a plant creature.
Benefit: You can spend a wild shape to gain immunity to critical hits, poison, sleep, paralysis, polymorph, and stunning. You also gain great stability, which gives you a +8 bonus on checks to avoid being bull rushed or tripped. The effect remains for 10 minutes.
Source: Complete Divine, page 82.