western hemlock vs hemlock tree

western hemlock and hemlock tree both are nouns.

Word	Noun	Adjective	Verb	Adverb
western hemlock	Yes	No	No	No
hemlock tree	Yes	No	No	No

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

western hemlock: tall evergreen of western North America; commercially important timber tree
hemlock tree: an evergreen tree, could refer to any one of a genus of conifers named Tsuga

Other hypernyms of western hemlock include hemlock.

western hemlock (noun)	hemlock tree (noun)
tall evergreen of western North America; commercially important timber tree	an evergreen tree, could refer to any one of a genus of conifers named Tsuga