Western mountain ash vs Sorbus sitchensis

Western mountain ash and Sorbus sitchensis both are nouns.

Word	Noun	Adjective	Verb	Adverb
Western mountain ash	Yes	No	No	No
Sorbus sitchensis	Yes	No	No	No

As nouns, Western mountain ash and Sorbus sitchensis are synonyms defined as:

Western mountain ash and Sorbus sitchensis: an ash of the western coast of North America

Western mountain ash (noun)	Sorbus sitchensis (noun)
an ash of the western coast of North America	an ash of the western coast of North America