plum vs fruit tree

plum and fruit tree both are nouns.

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

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

plum: any of several trees producing edible oval fruit having a smooth skin and a single hard stone
fruit tree: tree bearing edible fruit

plum (noun)	fruit tree (noun)
a highly desirable position or assignment	tree bearing edible fruit
any of numerous varieties of small to medium-sized round or oval fruit having a smooth skin and a single pit
any of several trees producing edible oval fruit having a smooth skin and a single hard stone

plum (adverb)	fruit tree (adverb)
exactly
completely; used as intensifiers