Betula nigra vs birch tree

Betula nigra and birch tree both are nouns.

Word	Noun	Adjective	Verb	Adverb
Betula nigra	Yes	No	No	No
birch tree	Yes	No	No	No

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

Betula nigra: 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 Betula nigra include birch.

Betula nigra (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