scrub oak vs oak tree

scrub oak and oak tree both are nouns.

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

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

scrub oak: any of various chiefly American small shrubby oaks often a dominant form on thin dry soils sometimes forming dense thickets
oak tree: a deciduous tree of the genus Quercus; has acorns and lobed leaves

Other hypernyms of scrub oak include oak.

scrub oak (noun)	oak tree (noun)
any of various chiefly American small shrubby oaks often a dominant form on thin dry soils sometimes forming dense thickets	a deciduous tree of the genus Quercus; has acorns and lobed leaves