holt vs understory

holt

noun
  • A small piece of woodland or a woody hill; a copse. 

  • The lair of an animal, especially of an otter. 

understory

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

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