The official GemStone IV encyclopedia.
The undead tree spirit resides among its living brethren, barely distinguishable from them until it is awakened from its slumber. It resembles many different types of towering trees, for a tree spirit is able to take on the shape and appearance of the forest around it. Being spirit, though, it is not quite solid, not quite sharply defined, and its appearance shifts slightly as it moves. Many are fooled by a tree spirit's soft, soothing whispering, only to realize with horror that it is the preparation of a lethal spell.
Beware the tree spirit's ability to cast Earthen Fury (917), which can cause severe damage if you are improperly trained to defend against SMRv2 attacks. The tree spirit can also cast Call Wind (912), which can induce hard roundtime and reduce stance considerably.
Tree spirits will decay very quickly when killed (approximately upon their next turn), making it tricky to obtain loot if you are under hard roundtime (i.e. from swinging a melee weapon). This may make hunting tree spirits more suitable for spellcasters, who primarily undergo cast roundtime during battle.