black willow vs willow tree

black willow and willow tree both are nouns.

Word	Noun	Adjective	Verb	Adverb
black willow	Yes	No	No	No
willow tree	Yes	No	No	No

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

black willow: 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 black willow include willow.

black willow (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