Salix nigra vs willow tree

Salix nigra and willow tree both are nouns.

Word	Noun	Adjective	Verb	Adverb
Salix nigra	Yes	No	No	No
willow tree	Yes	No	No	No

As nouns, willow tree is a hypernym of Salix nigra; that is, willow tree is a word with a broader meaning than Salix nigra:

Salix nigra: North American shrubby willow having dark bark and linear leaves growing close to streams and lakes
willow tree: any of numerous deciduous trees and shrubs of the genus Salix

Other hypernyms of Salix nigra include willow.

Salix nigra (noun)	willow tree (noun)
North American shrubby willow having dark bark and linear leaves growing close to streams and lakes	any of numerous deciduous trees and shrubs of the genus Salix