black ash vs ash tree

black ash and ash tree both are nouns.

Word	Noun	Adjective	Verb	Adverb
black ash	Yes	No	No	No
ash tree	Yes	No	No	No

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

black ash: 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 black ash include ash.

black ash (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