white walnut vs walnut tree

white walnut and walnut tree both are nouns.

Word	Noun	Adjective	Verb	Adverb
white walnut	Yes	No	No	No
walnut tree	Yes	No	No	No

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

white walnut: North American walnut tree having light-brown wood and edible nuts; source of a light-brown dye
walnut tree: any of various trees of the genus Juglans

Other hypernyms of white walnut include walnut.

white walnut (noun)	walnut tree (noun)
North American walnut tree having light-brown wood and edible nuts; source of a light-brown dye	any of various trees of the genus Juglans