Juglans regia vs walnut tree

Juglans regia and walnut tree both are nouns.

Word	Noun	Adjective	Verb	Adverb
Juglans regia	Yes	No	No	No
walnut tree	Yes	No	No	No

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

Juglans regia: Eurasian walnut valued for its large edible nut and its hard richly figured wood; widely cultivated
walnut tree: any of various trees of the genus Juglans

Other hypernyms of Juglans regia include walnut.

Juglans regia (noun)	walnut tree (noun)
Eurasian walnut valued for its large edible nut and its hard richly figured wood; widely cultivated	any of various trees of the genus Juglans