Sitka willow vs willow tree

Sitka willow and willow tree both are nouns.

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

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

Sitka willow: small shrubby tree of western North America (Alaska to Oregon)
willow tree: any of numerous deciduous trees and shrubs of the genus Salix

Other hypernyms of Sitka willow include willow.

Sitka willow (noun)	willow tree (noun)
small shrubby tree of western North America (Alaska to Oregon)	any of numerous deciduous trees and shrubs of the genus Salix