shoe tree vs shoetree

shoe tree

noun
  • A device approximating the shape of a foot that is placed inside a shoe when it is being stored to preserve its shape and extend the life of the shoe. 

shoetree

noun
  • A foot-shaped device placed in a shoe to maintain its shape when not being worn. 

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