robusta coffee vs coffee tree

robusta coffee and coffee tree both are nouns.

Word	Noun	Adjective	Verb	Adverb
robusta coffee	Yes	No	No	No
coffee tree	Yes	No	No	No

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

robusta coffee: native to West Africa but grown in Java and elsewhere; resistant to coffee rust
coffee tree: any of several small trees and shrubs native to the tropical Old World yielding coffee beans

Other hypernyms of robusta coffee include coffee.

robusta coffee (noun)	coffee tree (noun)
native to West Africa but grown in Java and elsewhere; resistant to coffee rust	any of several small trees and shrubs native to the tropical Old World yielding coffee beans