big-tree plum vs plum tree

big-tree plum and plum tree both are nouns.

Word	Noun	Adjective	Verb	Adverb
big-tree plum	Yes	No	No	No
plum tree	Yes	No	No	No

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

big-tree plum: small tree of southwestern United States having purplish-red fruit sometimes cultivated as an ornamental for its large leaves
plum tree: any of several trees producing edible oval fruit having a smooth skin and a single hard stone

Other hypernyms of big-tree plum include plum.

big-tree plum (noun)	plum tree (noun)
small tree of southwestern United States having purplish-red fruit sometimes cultivated as an ornamental for its large leaves	any of several trees producing edible oval fruit having a smooth skin and a single hard stone