rock elm vs Ulmus americana

rock elm and Ulmus americana both are nouns.

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

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

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

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

rock elm (noun)	Ulmus americana (noun)
tall widely distributed elm of eastern North America	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