Prunus nigra vs plum tree

Prunus nigra and plum tree both are nouns.

Word	Noun	Adjective	Verb	Adverb
Prunus nigra	Yes	No	No	No
plum tree	Yes	No	No	No

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

Prunus nigra: small tree native to northeastern North America having oblong orange-red fruit
plum tree: any of several trees producing edible oval fruit having a smooth skin and a single hard stone

Other hypernyms of Prunus nigra include plum.

Prunus nigra (noun)	plum tree (noun)
small tree native to northeastern North America having oblong orange-red fruit	any of several trees producing edible oval fruit having a smooth skin and a single hard stone