Japanese oak vs oak tree

Japanese oak and oak tree both are nouns.

Word	Noun	Adjective	Verb	Adverb
Japanese oak	Yes	No	No	No
oak tree	Yes	No	No	No

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

Japanese oak: oak with moderately light fine-grained wood; Japan
oak tree: a deciduous tree of the genus Quercus; has acorns and lobed leaves

Other hypernyms of Japanese oak include oak.

Japanese oak (noun)	oak tree (noun)
small evergreen tree of China and Japan	a deciduous tree of the genus Quercus; has acorns and lobed leaves
oak with moderately light fine-grained wood; Japan