Salix fragilis vs willow tree

Salix fragilis and willow tree both are nouns.

Word	Noun	Adjective	Verb	Adverb
Salix fragilis	Yes	No	No	No
willow tree	Yes	No	No	No

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

Salix fragilis: 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 Salix fragilis include willow.

Salix fragilis (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