The Collaborative International Dictionary of English v.0.48:

Fir tree \Fir" tree`\ (f[~e]r" tr[=e]`).
   See Fir.
   [1913 Webster]

WordNet (r) 3.0 (2006):

fir tree
    n 1: any of various evergreen trees of the genus Abies; chiefly
         of upland areas [syn: fir, fir tree, true fir]