rock elm vs elm tree

rock elm and elm tree both are nouns.

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

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

rock elm: tall widely distributed elm of eastern North America
elm tree: any of various trees of the genus Ulmus: important timber or shade trees

Other hypernyms of rock elm include elm.

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

rock 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 rock elm include elm.

rock elm (noun)	elm tree (noun)
tall widely distributed elm of eastern North America	any of various trees of the genus Ulmus: important timber or shade trees
large ornamental tree with graceful gradually spreading branches common in eastern North America