white willow vs willow tree

white willow and willow tree both are nouns.

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

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

white willow: large willow tree of Eurasia and North Africa having greyish canescent leaves and grey bark
willow tree: any of numerous deciduous trees and shrubs of the genus Salix

Other hypernyms of white willow include willow.

white willow (noun)	willow tree (noun)
large willow tree of Eurasia and North Africa having greyish canescent leaves and grey bark	any of numerous deciduous trees and shrubs of the genus Salix