Salix herbacea vs willow tree

Salix herbacea and willow tree both are nouns.

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

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

Salix herbacea: widely distributed boreal shrubby willow with partially underground creeping stems and bright green glossy leaves
willow tree: any of numerous deciduous trees and shrubs of the genus Salix

Other hypernyms of Salix herbacea include willow.

Salix herbacea (noun)	willow tree (noun)
widely distributed boreal shrubby willow with partially underground creeping stems and bright green glossy leaves	any of numerous deciduous trees and shrubs of the genus Salix