pussy willow vs willow tree

pussy willow and willow tree both are nouns.

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

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

pussy willow: small willow of eastern North America having greyish leaves and silky catkins that come before the leaves
willow tree: any of numerous deciduous trees and shrubs of the genus Salix

Other hypernyms of pussy willow include willow.

pussy willow (noun)	willow tree (noun)
small willow of eastern North America having greyish leaves and silky catkins that come before the leaves	any of numerous deciduous trees and shrubs of the genus Salix
much-branched Old World willow having large catkins and relatively large broad leaves