balsam willow vs willow tree

balsam willow and willow tree both are nouns.

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

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

balsam willow: small shrubby tree of eastern North America having leaves exuding an odor of balsam when crushed
willow tree: any of numerous deciduous trees and shrubs of the genus Salix

Other hypernyms of balsam willow include willow.

balsam willow (noun)	willow tree (noun)
small shrubby tree of eastern North America having leaves exuding an odor of balsam when crushed	any of numerous deciduous trees and shrubs of the genus Salix