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The Tree People were antagonists in the early Casper cartoon "Boo Moon" and were a presumably alien race and enemies of the Moon People - true to their name, they take the form of vaguely humanoid trees and do not speak nor seem to reason much beyond mindless attacking.
In fact in their sole appearance the Tree People storm the Moon People's kingdom and begin to capture the tiny aliens while causing havoc - Casper, who was visiting the moon at the time, witnessed this and realized the Moon People needed help against the Tree People (who were much too large for the tiny aliens to fight).
Thus Casper used his powers as a ghost to disappear under the ground and root the Tree People (quite literally) via tieing their "feet" together - immobilzing them and stopping their attack.
It is presumed the Tree People were either forced to leave the Moon People alone after this or remain prisoners but it is never resolved, instead the cartoon ends with Casper being rewarded for saving the Moon People by being knighted by the Moon People's king - the fate of the Tree People is unknown.