black cottonwood vs cottonwood

black cottonwood and cottonwood both are nouns.

Word	Noun	Adjective	Verb	Adverb
black cottonwood	Yes	No	No	No
cottonwood	Yes	No	No	No

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

black cottonwood: cottonwood of western North America with dark green leaves shining above and rusty or silvery beneath
cottonwood: any of several North American trees of the genus Populus having a tuft of cottony hairs on the seed

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

black cottonwood: North American poplar with large rounded scalloped leaves and brownish bark and wood
cottonwood: any of several North American trees of the genus Populus having a tuft of cottony hairs on the seed

black cottonwood (noun)	cottonwood (noun)
cottonwood of western North America with dark green leaves shining above and rusty or silvery beneath	any of several North American trees of the genus Populus having a tuft of cottony hairs on the seed
North American poplar with large rounded scalloped leaves and brownish bark and wood	American basswood of the Allegheny region