white willow vs willow

white willow and willow both are nouns.

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

As nouns, willow is a hypernym of white willow; that is, willow 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: any of numerous deciduous trees and shrubs of the genus Salix

Other hypernyms of white willow include willow tree.

white willow (noun)	willow (noun)
large willow tree of Eurasia and North Africa having greyish canescent leaves and grey bark	a textile machine having a system of revolving spikes for opening and cleaning raw textile fibers
	any of numerous deciduous trees and shrubs of the genus Salix