sweet orange vs orange tree

sweet orange and orange tree both are nouns.

Word	Noun	Adjective	Verb	Adverb
sweet orange	Yes	No	No	No
orange tree	Yes	No	No	No

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

sweet orange: probably native to southern China; widely cultivated as source of table and juice oranges
orange tree: any citrus tree bearing oranges

Other hypernyms of sweet orange include orange.

sweet orange (noun)	orange tree (noun)
orange with sweet juicy pulp; often has a thin skin	any citrus tree bearing oranges
probably native to southern China; widely cultivated as source of table and juice oranges