assart vs open space

assart

noun
  • Forest land cleared for agriculture. 

verb
  • To clear forest land for agriculture; remove stumps. 

open space

noun
  • An open-air area, such as a park or public garden with no (or very few) buildings, often in an urban area and designated by a local authority or government as such. 

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