Question: Which is a common name for a tree species, Aspen or Clethra? 

Context paragraphs:
Orchid tree: Orchid tree is a common name for several tree species, including:

List of plants known as tōtara: Tōtara is a Māori common name for the tree species "Podocarpus totara".  The term may also refer to other similar species in the genus "Podocarpus":

Populus tremuloides: Populus tremuloides is a deciduous tree native to cooler mountainous areas of North America, one of several species referred to by the common name aspen.  It is commonly called quaking aspen, trembling aspen, American aspen, Quakies, mountain or golden aspen, trembling poplar, white poplar, popple, and even more names.  The trees have tall trunks, up to 25 meters (82 feet) tall, with smooth pale bark, scarred with black.  The glossy green leaves, dull beneath, become golden to yellow, rarely red, in autumn.  The species often propagates through its roots to form large clonal groves originating from a shared root system.  These roots are not rhizomes, as new growth develops from adventitious buds on the parent root system (the ortet).

Attalea phalerata: Attalea phalerata is a species of palm tree known by the English common name urucuri palm, the Portuguese common name urucurizeiro, and the Spanish common name shapaja.  Other common names include motacu and bacuri.  It is native to Brazil, Bolivia, Paraguay, and Peru, where it grows along southern and western Amazonia.  It is the most common palm tree on the Pantanal.

Populus balsamifera: Populus balsamifera, commonly called balsam poplar, bam, bamtree, eastern balsam-poplar, hackmatack, tacamahac poplar, tacamahaca, is a tree species in the balsam poplar species group in the poplar genus, "Populus."  The genus name "Populus" is from the Latin for poplar, and the specific epithet "balsamifera" from Latin for "balsam-bearing".  Other common names for the species include heartleaf balsam poplar, and Ontario balsam poplar.  The black cottonwood, "Populus trichocarpa", is sometimes considered a subspecies of "P. balsamifera" and may lend its common name to this species, although the black poplars and cottonwoods of "Populus" sect.  "Aigeiros" are not closely related.

Quebracho tree: Quebracho (also known as Luciano malo) is a common name in Spanish to describe very hard (density 1.15 - 1.35) wood tree species.  The etymology of the name derived from "quiebrahacha", or "quebrar hacha", meaning "axe-breaker".

Eucalyptus morrisbyi: Eucalyptus morrisbyi is a rare and endangered tree species endemic to south-eastern Tasmania.  Its common name is Morrisby's Gum.  It has two natural populations on the eastern shore of the Derwent River valley.  The limited range of "E. morrisbyi" means effective management is required to maintain or expand the distribution of this species.

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.

Toxicodendron vernicifluum: Toxicodendron vernicifluum (formerly "Rhus verniciflua"), also known by the common name Chinese lacquer tree, is an Asian tree species of genus "Toxicodendron" (formerly "Rhus") native to China and the Indian subcontinent, and cultivated in regions of China, Korea and Japan.  Other common names include "Japanese lacquer tree", "Japanese sumac", and "varnish tree".  The trees are cultivated and tapped for their toxic sap, which is used as a highly durable lacquer to make Chinese, Japanese, and Korean lacquerware.

Clethra: Clethra is a genus of flowering shrubs or small trees described as a genus by Linnaeus in 1753.

Supporting paragraphs: Aspen Clethra
Answer: Aspen