American gray birch vs grey birch

American gray birch and grey birch both are nouns.

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

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

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

American gray birch (noun)	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