silver fir vs fir tree

silver fir and fir tree both are nouns.

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

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

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

Other hypernyms of silver fir include fir, true fir.

silver fir (noun)	fir tree (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