water locust vs locust tree

water locust and locust tree both are nouns.

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

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

water locust: honey locust of swamps and bottomlands of southern United States having short oval pods; yields dark heavy wood
locust tree: any of various hardwood trees of the family Leguminosae

Other hypernyms of water locust include locust.

water locust (noun)	locust tree (noun)
honey locust of swamps and bottomlands of southern United States having short oval pods; yields dark heavy wood	any of various hardwood trees of the family Leguminosae