American elm vs Ulmus americana

American elm and Ulmus americana both are nouns.

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

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

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

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

American 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