water hickory vs hickory tree

water hickory and hickory tree both are nouns.

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

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

water hickory: hickory of southern United States having many narrow leaflets and rather bitter nuts
hickory tree: American hardwood tree bearing edible nuts

Other hypernyms of water hickory include hickory.

water hickory (noun)	hickory tree (noun)
hickory of southern United States having many narrow leaflets and rather bitter nuts	American hardwood tree bearing edible nuts