Prunus mexicana vs plum tree

Prunus mexicana and plum tree both are nouns.

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

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

Prunus mexicana: small tree of southwestern United States having purplish-red fruit sometimes cultivated as an ornamental for its large leaves
plum tree: any of several trees producing edible oval fruit having a smooth skin and a single hard stone

Other hypernyms of Prunus mexicana include plum.

Prunus mexicana (noun)	plum tree (noun)
small tree of southwestern United States having purplish-red fruit sometimes cultivated as an ornamental for its large leaves	any of several trees producing edible oval fruit having a smooth skin and a single hard stone