swamp willow vs willow tree

swamp willow and willow tree both are nouns.

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

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

swamp 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 swamp willow include willow.

swamp 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