bark vs natural covering

bark and natural covering both are nouns.

Word	Noun	Adjective	Verb	Adverb
bark	Yes	No	Yes	No
natural covering	Yes	No	No	No

As nouns, natural covering is a hypernym of bark; that is, natural covering is a word with a broader meaning than bark:

bark: tough protective covering of the woody stems and roots of trees and other woody plants
natural covering: a natural object that covers or envelops

Other hypernyms of bark include cover, covering.

bark (noun)	natural covering (noun)
a sailing ship with 3 (or more) masts	a natural object that covers or envelops
the sound made by a dog
a noise resembling the bark of a dog
tough protective covering of the woody stems and roots of trees and other woody plants