Which is a common name for a tree species, Aspen or Clethra? 
Aspen: Aspen is a common name for certain tree species; some, but not all, are classified by botanists in the section "Populus", of the "Populus" genus. Clethra: Clethra is a genus of flowering shrubs or small trees described as a genus by Linnaeus in 1753.
Aspen