dwarf elm vs elm tree

dwarf elm and elm tree both are nouns.

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

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

dwarf elm: fast-growing shrubby Asian tree naturalized in United States for shelter or ornament
elm tree: any of various trees of the genus Ulmus: important timber or shade trees

Other hypernyms of dwarf elm include elm.

dwarf elm (noun)	elm tree (noun)
fast-growing shrubby Asian tree naturalized in United States for shelter or ornament	any of various trees of the genus Ulmus: important timber or shade trees