mountain birch vs Western paper birch

mountain birch and Western paper birch both are nouns.

Word	Noun	Adjective	Verb	Adverb
mountain birch	Yes	No	No	No
Western paper birch	Yes	No	No	No

As nouns, mountain birch and Western paper birch are synonyms defined as:

mountain birch and Western paper birch: birch of western United States resembling the paper birch but having brownish bark

Other synonyms of mountain birch include Betula fontinalis, swamp birch, water birch, Western birch.

mountain birch (noun)	Western paper birch (noun)
birch of western United States resembling the paper birch but having brownish bark	birch of western United States resembling the paper birch but having brownish bark