water birch vs birch tree

water birch and birch tree both are nouns.

Word	Noun	Adjective	Verb	Adverb
water birch	Yes	No	No	No
birch tree	Yes	No	No	No

As nouns, birch tree is a hypernym of water birch; that is, birch tree is a word with a broader meaning than water birch:

water birch: birch of western United States resembling the paper birch but having brownish bark
birch tree: any betulaceous tree or shrub of the genus Betula having a thin peeling bark

Other hypernyms of water birch include birch.

water birch (noun)	birch tree (noun)
birch of western United States resembling the paper birch but having brownish bark	any betulaceous tree or shrub of the genus Betula having a thin peeling bark