common plum vs plum tree

common plum and plum tree both are nouns.

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

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

common plum: any of various widely distributed plums grown in the cooler temperate areas
plum tree: any of several trees producing edible oval fruit having a smooth skin and a single hard stone

Other hypernyms of common plum include plum.

common plum (noun)	plum tree (noun)
any of various widely distributed plums grown in the cooler temperate areas	any of several trees producing edible oval fruit having a smooth skin and a single hard stone