grey birch vs American gray birch

grey birch and American gray birch both are nouns.

Word	Noun	Adjective	Verb	Adverb
grey birch	Yes	No	No	No
American gray birch	Yes	No	No	No

As nouns, grey birch and American gray birch are synonyms defined as:

grey birch and American gray birch: medium-sized birch of eastern North America having white or pale grey bark and valueless wood; occurs often as a second-growth forest tree

Other synonyms of grey birch include American grey birch, Betula populifolia, gray birch.

grey birch (noun)	American gray birch (noun)
medium-sized birch of eastern North America having white or pale grey bark and valueless wood; occurs often as a second-growth forest tree	medium-sized birch of eastern North America having white or pale grey bark and valueless wood; occurs often as a second-growth forest tree