brittle willow vs willow tree

brittle willow and willow tree both are nouns.

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

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

brittle willow: large willow tree with stiff branches that are easily broken
willow tree: any of numerous deciduous trees and shrubs of the genus Salix

Other hypernyms of brittle willow include willow.

brittle willow (noun)	willow tree (noun)
large willow tree with stiff branches that are easily broken	any of numerous deciduous trees and shrubs of the genus Salix