Sierra plum vs plum tree

Sierra plum and plum tree both are nouns.

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

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

Sierra 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 Sierra plum include plum.

Sierra 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