cottonwood vs poplar tree

cottonwood and poplar tree both are nouns.

Word	Noun	Adjective	Verb	Adverb
cottonwood	Yes	No	No	No
poplar tree	Yes	No	No	No

As nouns, poplar tree is a hypernym of cottonwood; that is, poplar tree is a word with a broader meaning than cottonwood:

cottonwood: any of several North American trees of the genus Populus having a tuft of cottony hairs on the seed
poplar tree: any of numerous trees of north temperate regions having light soft wood and flowers borne in catkins

Other hypernyms of cottonwood include poplar.

cottonwood (noun)	poplar tree (noun)
any of several North American trees of the genus Populus having a tuft of cottony hairs on the seed	any of numerous trees of north temperate regions having light soft wood and flowers borne in catkins
American basswood of the Allegheny region