gray birch vs American grey birch

gray birch and American grey birch both are nouns.

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

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

gray birch and American grey 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 gray birch include American gray birch, Betula populifolia, grey birch.

gray birch (noun)	American grey 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