wild plum tree vs plum tree

wild plum tree and plum tree both are nouns.

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

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

wild plum tree: an uncultivated plum tree or shrub
plum tree: any of several trees producing edible oval fruit having a smooth skin and a single hard stone

Other hypernyms of wild plum tree include plum.

wild plum tree (noun)	plum tree (noun)
an uncultivated plum tree or shrub	any of several trees producing edible oval fruit having a smooth skin and a single hard stone