Fraxinus nigra vs ash tree

Fraxinus nigra and ash tree both are nouns.

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

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

Fraxinus nigra: vigorous spreading North American tree having dark brown heavy wood; leaves turn gold in autumn
ash tree: any of various deciduous pinnate-leaved ornamental or timber trees of the genus Fraxinus

Other hypernyms of Fraxinus nigra include ash.

Fraxinus nigra (noun)	ash tree (noun)
vigorous spreading North American tree having dark brown heavy wood; leaves turn gold in autumn	any of various deciduous pinnate-leaved ornamental or timber trees of the genus Fraxinus