mountain fern vs fern

mountain fern and fern both are nouns.

Word	Noun	Adjective	Verb	Adverb
mountain fern	Yes	No	No	No
fern	Yes	No	No	No

As nouns, fern is a hypernym of mountain fern; that is, fern is a word with a broader meaning than mountain fern:

mountain fern: common European mountain fern having fragrant lemon or balsam scented fronds
fern: any of numerous flowerless and seedless vascular plants having true roots from a rhizome and fronds that uncurl upward; reproduce by spores

mountain fern (noun)	fern (noun)
common European mountain fern having fragrant lemon or balsam scented fronds	any of numerous flowerless and seedless vascular plants having true roots from a rhizome and fronds that uncurl upward; reproduce by spores