fir vs underbrush

fir

noun
  • Wood of such trees. 

  • Any pinaceous conifer of related genera, especially a Douglas fir (Pseudotsuga) or a Scots pine (Pinus sylvestris). 

  • A conifer of the genus Abies. 

underbrush

noun
  • The small trees and other plants that clutter the floor of a forest. 

verb
  • To clear (an area) of underbrush. 

  • To work among the underbrush. 

How often have the words fir and underbrush occurred in a corpus of books? (source: Google Ngram Viewer )