Quercus nigra vs oak tree

Quercus nigra and oak tree both are nouns.

Word	Noun	Adjective	Verb	Adverb
Quercus nigra	Yes	No	No	No
oak tree	Yes	No	No	No

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

Quercus nigra: relatively tall deciduous water oak of southeastern United States often cultivated as a shade tree; thrives in wet soil
oak tree: a deciduous tree of the genus Quercus; has acorns and lobed leaves

Other hypernyms of Quercus nigra include oak.

Quercus nigra (noun)	oak tree (noun)
relatively tall deciduous water oak of southeastern United States often cultivated as a shade tree; thrives in wet soil	a deciduous tree of the genus Quercus; has acorns and lobed leaves