sweet lemon vs lemon tree

sweet lemon and lemon tree both are nouns.

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

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

sweet lemon: lemon tree having fruit with a somewhat insipid sweetish pulp
lemon tree: a small evergreen tree that originated in Asia but is widely cultivated for its fruit

Other hypernyms of sweet lemon include Citrus limon, lemon.

sweet lemon (noun)	lemon tree (noun)
lemon tree having fruit with a somewhat insipid sweetish pulp	a small evergreen tree that originated in Asia but is widely cultivated for its fruit