genus Betula vs birch tree

genus Betula and birch tree both are nouns.

Word	Noun	Adjective	Verb	Adverb
genus Betula	Yes	No	No	No
birch tree	Yes	No	No	No

genus Betula (noun)	birch tree (noun)
a genus of trees of the family Betulaceae (such as birches)	any betulaceous tree or shrub of the genus Betula having a thin peeling bark