bitter hickory vs hickory tree

bitter hickory and hickory tree both are nouns.

Word	Noun	Adjective	Verb	Adverb
bitter hickory	Yes	No	No	No
hickory tree	Yes	No	No	No

As nouns, hickory tree is a hypernym of bitter hickory; that is, hickory tree is a word with a broader meaning than bitter hickory:

bitter hickory: hickory of the eastern United States having a leaves with 7 or 9 leaflets and thin-shelled very bitter nuts
hickory tree: American hardwood tree bearing edible nuts

Other hypernyms of bitter hickory include hickory.

bitter hickory (noun)	hickory tree (noun)
hickory of the eastern United States having a leaves with 7 or 9 leaflets and thin-shelled very bitter nuts	American hardwood tree bearing edible nuts