Juglans nigra vs black walnut

Juglans nigra

black walnut

noun
  • The nuts of the aforementioned tree. 

  • The wood of the aforementioned species. 

  • Juglans nigra, a tree native to eastern North America. 

How often have the words Juglans nigra and black walnut occurred in a corpus of books? (source: Google Ngram Viewer )