bracken vs understory

bracken

noun
  • An area of countryside heavily populated by this fern. 

  • Any of several coarse ferns, of the genus Pteridium, that form dense thickets; often poisonous to livestock. 

understory

noun
  • The layer of plants that grow in the shade of the canopy of a forest. 

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