Rhamnus frangula vs alder buckthorn

Rhamnus frangula

alder buckthorn

noun
  • A deciduous shrub, Frangula alnus, native to Europe and neighboring areas of Asia and Africa, bearing glossy leaves, flowers in sessile umbels, and inedible red berries that turn black when ripe, and historically used as a laxative and to produce dyes. 

How often have the words Rhamnus frangula and alder buckthorn occurred in a corpus of books? (source: Google Ngram Viewer )