Justin Tree was a man who was transformed into a tree by the Evil Magician Trent's.
Justin Tree was a resident of North Village during the rise of the Evil Magician Trent. During Trent's march on the throne in 1022, he found Justin to be a nuisence and used his powers to transform Justin into a tree.  When Trent returned to Xanth and became king, Justin refused to be transformed back into a man.
Justin retained some human-like features, including leaves shaped like hands and bark colored like human skin. Located along the edge of North Village, Justin had considerable time to think. This made popular with nearby villagers as a source of advice. His experiences as both man and tree gave him a perspective and objectivity well suited for this role. Rumors were that Justin preferred being a tree, as he was more stately and less threatening than he was as a man. 
Justin was eventually transformed back into a man after having a Quest by sharing the mind of Breanna of the Black Wave. After she turned eighteen, he married Breanna. They are now in charge of Castle Zombie.
Justin's talent was the ability of voice projection. He could project his voice at a distance without the use of his vocal cords.