Fraxinus Americana vs ash tree

Fraxinus Americana and ash tree both are nouns.

Word	Noun	Adjective	Verb	Adverb
Fraxinus Americana	Yes	No	No	No
ash tree	Yes	No	No	No

As nouns, ash tree is a hypernym of Fraxinus Americana; that is, ash tree is a word with a broader meaning than Fraxinus Americana:

Fraxinus Americana: spreading American ash with leaves pale green or silvery beneath and having hard brownish wood
ash tree: any of various deciduous pinnate-leaved ornamental or timber trees of the genus Fraxinus

Other hypernyms of Fraxinus Americana include ash.

Fraxinus Americana (noun)	ash tree (noun)
spreading American ash with leaves pale green or silvery beneath and having hard brownish wood	any of various deciduous pinnate-leaved ornamental or timber trees of the genus Fraxinus