mountain alder vs white alder

mountain alder and white alder both are nouns.

Word	Noun	Adjective	Verb	Adverb
mountain alder	Yes	No	No	No
white alder	Yes	No	No	No

As nouns, mountain alder and white alder are synonyms defined as:

Other synonyms of mountain alder include Alnus rhombifolia.

mountain alder (noun)	white alder (noun)
small shrubby maple of eastern North America; scarlet in autumn	tree of western United States
tree of western United States	shrub of eastern and southern coastal United States having beautiful racemes of spice-scented white flowers