The living tree appeared in Holidays of Future Passed.
Throughout the years, the tree had held the Simpson treehouse. When one pine tree revealed the sentience of all other trees to mankind, the Simpson backyard tree was among those that revealed its existence. It now helps occupants of the treehouse back down to the ground.
Not much is known about the tree, but it appears bitter from taking orders and seems upset that the pine tree revealed trees' sentience.
He appears in all the episodes Bart's treehouse appears in.