The forest ruled by Treebeard in J.R.R. Tolkien’s The Lord of the Rings is Fangorn Forest.
Fangorn lies at the eastern end of the Eregion region and near Isengard. It is an ancient woodland inhabited by Ents, tree-like beings who guard and shepherd the forest. Treebeard, whose Entish name is Fangorn, is the oldest of the Ents encountered by the story’s characters.
Merry and Pippin meet Treebeard after escaping from the Orcs who captured them. Their conversations help persuade the Ents to attack Isengard, where Saruman has destroyed trees to support his war machine. Fangorn is sometimes confused with Mirkwood, another important forest in Tolkien’s legendarium, but Mirkwood lies far to the northeast and has a different history.