water elm vs Ulmus americana

water elm and Ulmus americana both are nouns.

Word	Noun	Adjective	Verb	Adverb
water elm	Yes	No	No	No
Ulmus americana	Yes	No	No	No

As nouns, water elm and Ulmus americana are synonyms defined as:

water elm and Ulmus americana: large ornamental tree with graceful gradually spreading branches common in eastern North America

Other synonyms of water elm include American elm, rock elm, white elm.

water elm (noun)	Ulmus americana (noun)
large ornamental tree with graceful gradually spreading branches common in eastern North America	large ornamental tree with graceful gradually spreading branches common in eastern North America
Eurasian elm closely resembling the American elm; thrives in a moist environment