black willow vs Salix nigra

black willow and Salix nigra both are nouns.

Word	Noun	Adjective	Verb	Adverb
black willow	Yes	No	No	No
Salix nigra	Yes	No	No	No

As nouns, black willow and Salix nigra are synonyms defined as:

black willow and Salix nigra: North American shrubby willow having dark bark and linear leaves growing close to streams and lakes

Other synonyms of black willow include swamp willow.

black willow (noun)	Salix nigra (noun)
North American shrubby willow having dark bark and linear leaves growing close to streams and lakes	North American shrubby willow having dark bark and linear leaves growing close to streams and lakes