eastern hemlock vs hemlock tree

eastern hemlock and hemlock tree both are nouns.

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

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

eastern hemlock: common forest tree of the eastern United States and Canada; used especially for pulpwood
hemlock tree: an evergreen tree, could refer to any one of a genus of conifers named Tsuga

Other hypernyms of eastern hemlock include hemlock.

eastern hemlock (noun)	hemlock tree (noun)
common forest tree of the eastern United States and Canada; used especially for pulpwood	an evergreen tree, could refer to any one of a genus of conifers named Tsuga