Sitka willow vs willow

Sitka willow and willow both are nouns.

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

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

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

Other hypernyms of Sitka willow include willow tree.

Sitka willow (noun)	willow (noun)
small shrubby tree of western North America (Alaska to Oregon)	a textile machine having a system of revolving spikes for opening and cleaning raw textile fibers
	any of numerous deciduous trees and shrubs of the genus Salix