apple tree vs apple

apple tree and apple both are nouns.

Word	Noun	Adjective	Verb	Adverb
apple tree	Yes	No	No	No
apple	Yes	No	No	No

As nouns, apple is a hyponym of apple tree; that is, apple is a word with a more specific, narrower meaning than apple tree:

apple tree: any tree of the genus Malus especially those bearing firm rounded edible fruits
apple: native Eurasian tree widely cultivated in many varieties for its firm rounded edible fruits

apple tree (noun)	apple (noun)
any tree of the genus Malus especially those bearing firm rounded edible fruits	fruit with red or yellow or green skin and sweet to tart crisp whitish flesh
	native Eurasian tree widely cultivated in many varieties for its firm rounded edible fruits