Pacific plum vs plum tree

Pacific plum and plum tree both are nouns.

Word	Noun	Adjective	Verb	Adverb
Pacific plum	Yes	No	No	No
plum tree	Yes	No	No	No

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

Pacific plum: shrub of the Pacific coast of the United States bearing small red insipid fruit
plum tree: any of several trees producing edible oval fruit having a smooth skin and a single hard stone

Other hypernyms of Pacific plum include plum.

Pacific plum (noun)	plum tree (noun)
shrub of the Pacific coast of the United States bearing small red insipid fruit	any of several trees producing edible oval fruit having a smooth skin and a single hard stone