silver willow vs willow tree

silver willow and willow tree both are nouns.

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

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

silver willow: North American willow with greyish silky pubescent leaves that usually blacken in drying
willow tree: any of numerous deciduous trees and shrubs of the genus Salix

Other hypernyms of silver willow include willow.

silver willow (noun)	willow tree (noun)
North American willow with greyish silky pubescent leaves that usually blacken in drying	any of numerous deciduous trees and shrubs of the genus Salix