water elm vs elm tree

water elm and elm tree both are nouns.

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

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

water elm: large ornamental tree with graceful gradually spreading branches common in eastern North America
elm tree: any of various trees of the genus Ulmus: important timber or shade trees

Other hypernyms of water elm include elm.

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

water elm: Eurasian elm closely resembling the American elm; thrives in a moist environment
elm tree: any of various trees of the genus Ulmus: important timber or shade trees

Other hypernyms of water elm include elm.

water elm (noun)	elm tree (noun)
large ornamental tree with graceful gradually spreading branches common in eastern North America	any of various trees of the genus Ulmus: important timber or shade trees
Eurasian elm closely resembling the American elm; thrives in a moist environment