Salix tristis vs willow tree

Salix tristis and willow tree both are nouns.

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

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

Salix tristis: willow shrub of dry places in the eastern United States having long narrow leaves canescent beneath
willow tree: any of numerous deciduous trees and shrubs of the genus Salix

Other hypernyms of Salix tristis include willow.

Salix tristis (noun)	willow tree (noun)
willow shrub of dry places in the eastern United States having long narrow leaves canescent beneath	any of numerous deciduous trees and shrubs of the genus Salix