silver fir vs true fir

silver fir and true fir both are nouns.

Word	Noun	Adjective	Verb	Adverb
silver fir	Yes	No	No	No
true fir	Yes	No	No	No

As nouns, true fir is a hypernym of silver fir; that is, true fir is a word with a broader meaning than silver fir:

silver fir: any of various true firs having leaves white or silvery white beneath
true fir: any of various evergreen trees of the genus Abies; chiefly of upland areas

Other hypernyms of silver fir include fir, fir tree.

silver fir (noun)	true fir (noun)
any of various true firs having leaves white or silvery white beneath	any of various evergreen trees of the genus Abies; chiefly of upland areas