black birch vs birch tree

black birch and birch tree both are nouns.

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

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

black birch: birch of swamps and river bottoms throughout the eastern United States having reddish-brown bark
birch tree: any betulaceous tree or shrub of the genus Betula having a thin peeling bark

Other hypernyms of black birch include birch.

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

black 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 black birch include birch.

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