elm vs elm tree

elm and elm tree both are nouns.

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

As nouns, elm and elm tree are synonyms defined as:

elm and elm tree: any of various trees of the genus Ulmus: important timber or shade trees

elm (noun)	elm tree (noun)
any of various trees of the genus Ulmus: important timber or shade trees	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