Ulmus americana vs elm tree

Ulmus americana and elm tree both are nouns.

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

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

Ulmus americana: 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 Ulmus americana include elm.

Ulmus americana (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