downy birch vs birch tree

downy birch and birch tree both are nouns.

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

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

downy birch: European birch with dull white to pale brown bark and somewhat drooping hairy branches
birch tree: any betulaceous tree or shrub of the genus Betula having a thin peeling bark

Other hypernyms of downy birch include birch.

downy birch (noun)	birch tree (noun)
European birch with dull white to pale brown bark and somewhat drooping hairy branches	any betulaceous tree or shrub of the genus Betula having a thin peeling bark