Dryads are able to dwell inside trees or assume the form of trees by magical shape-shifting.
Source: Warhammer Armies Project: Unofficial 9th Edition (v2.2)
Tree AspectsURL Copied!
At the start of each round of close combat in which they are fighting, models with this special rule choose one of the following Tree Aspects, the effect of which lasts until the end of that turn or until a new Tree Aspect is chosen. This is done before Impact Hits are resolved. All Dryads in the same unit must choose the same Tree Aspect. Dryads cannot choose the same Tree Aspects in two consecutive rounds of combat.
Birch Aspect: Dryads which shift shape into their Birch aspect take on a silvery-white appearance like birch trees, with a mass of delicate yellow green or tawny yellow foliage for hair. They are extremely vicious and spiteful and lash the foe with their long fingers which cut into the flesh.
While in this Aspect, the Dryads gain +1 Attack.Oak Aspect: When Dryads shift shape into their Oak aspect, they become more robust and resilient to wounds. They assume a green or brown skin and their limbs thicken. Their hair becomes a mass of oak leaves and acorns.
While in this Aspect, the Dryads gain +1 Toughness.Willow Aspect: In their Willow aspect, Dryads change into creatures with green skin and extremely long yellowish green hair which hangs down around them like the leaves of the weeping willow tree. Their fingers extend in incredibly long, whip-like willow twigs. In close combat they entwine their fingers around the foe and grip his weapon making it difficult for him to wield it at all.
While in this Aspect, enemy models in base contact suffer -1 To Hit.