Betula nigra vs river birch

Betula nigra and river birch both are nouns.

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

As nouns, Betula nigra and river birch are synonyms defined as:

Betula nigra and river birch: birch of swamps and river bottoms throughout the eastern United States having reddish-brown bark

Other synonyms of Betula nigra include black birch, red birch.

Betula nigra (noun)	river birch (noun)
birch of swamps and river bottoms throughout the eastern United States having reddish-brown bark	birch of swamps and river bottoms throughout the eastern United States having reddish-brown bark