fir tree vs true fir

fir tree and true fir both are nouns.

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

As nouns, fir tree and true fir are synonyms defined as:

fir tree and true fir: any of various evergreen trees of the genus Abies; chiefly of upland areas

Other synonyms of fir tree include fir.

fir tree (noun)	true fir (noun)
any of various evergreen trees of the genus Abies; chiefly of upland areas	any of various evergreen trees of the genus Abies; chiefly of upland areas