From the Super Mario Wiki
Wise Wisterwood is a giant tree who oversees everything in the center of Flower Fields in Paper Mario. He is the elder of the fair land who watches over the area.
After Mario gets the fifth Star Spirit, Misstar; the volcano Mario and Kolarado is in starts to erupt as Misstar gets both of them out to safety. Kolarado is sad that the treasure chest in the volcano is said to be gone forever. Mario finds the treasure and gives it to Kolorado in return for the last Magical Seed. As before Mario goes back to Toad Town as usual, a Bub-ulb is crying because their home, Flower Fields was terrorized by Huff N. Puff. When Mario gives all the 4 Four Magical Seeds to Minh T. to plant it, the door appears as Mario and his partners can enter the place.
When Mario enters Flower Fields, he hears a voice in the area. The voice is actually a tree. The tree introduces himself to Mario as "Wise Wisterwood", as he is the eldest tree around Flower Fields. Wise Wister actually knows much about Mario as he mentions that his quest to save all the seven Star Spirits all around the world. He is surprised that Mario is attempting to get the sixth Star Spirit, Klevar in this place. He mentions there is a creature named Huff N. Puff who invaded the land with his troops not while ago. He then mentioned his recollection of a rumor he had heard that Huff N. Puff is currently holding someone captured; as he quickly understands that Mario was getting the sixth Star Spirit in the place. When Mario is attempting to defeat Huff N. Puff, he mentions that it is not easy as Huff N. Puff is located in the very sky as the plants call the place; Cloudy Climb as there is no way to reach there.
Since Mario can’t reach to Cloudy Climb, Wise Wisterwood quickly mentioned that there is a Magical Bean that can grow a tall beanstalk that would lead to Cloudy Climb. Mario collects the items , makes the sun shine again and then accesses Cloudy Climb, where he defeats Huff N. Puff and gains the Star Spirit, Klevar.
Later in the game, Wise Wisterwood writes Mario a letter. He also appears during the credits parade at the end of the game, where Merlon, Merluvlee, Merlee, Merlow, Merle and Madam Merlar use their Shaman magic to decorate and light up the tree.