sweet birch vs birch tree

sweet birch and birch tree both are nouns.

Word	Noun	Adjective	Verb	Adverb
sweet birch	Yes	No	No	No
birch tree	Yes	No	No	No

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

sweet birch: common birch of the eastern United States having spicy brown bark yielding a volatile oil and hard dark wood used for furniture
birch tree: any betulaceous tree or shrub of the genus Betula having a thin peeling bark

Other hypernyms of sweet birch include birch.

sweet birch (noun)	birch tree (noun)
common birch of the eastern United States having spicy brown bark yielding a volatile oil and hard dark wood used for furniture	any betulaceous tree or shrub of the genus Betula having a thin peeling bark