The People of the Trees were one of the native species of an Earth-like planet. They lived in trees and worshiped the Great Ones. The other native species, the Dascarians, considered them primitive savages.
The First Doctor befriended the People and protected them from losing their land. When they again needed his protection, the Fourth Doctor and Leela helped keep their land from Aulian Thorolis. (PROSE: People of the Trees)