white elm vs elm

white elm and elm both are nouns.

Word	Noun	Adjective	Verb	Adverb
white elm	Yes	No	No	No
elm	Yes	No	No	No

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

white elm: large ornamental tree with graceful gradually spreading branches common in eastern North America
elm: any of various trees of the genus Ulmus: important timber or shade trees

Other hypernyms of white elm include elm tree.

white elm (noun)	elm (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
	hard tough wood of an elm tree; used for e.g. implements and furniture