mountain pine vs pine tree

mountain pine and pine tree both are nouns.

Word	Noun	Adjective	Verb	Adverb
mountain pine	Yes	No	No	No
pine tree	Yes	No	No	No

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

mountain pine: low shrubby pine of central Europe with short bright green needles in bunches of two
pine tree: a coniferous tree

Other hypernyms of mountain pine include pine, true pine.

mountain pine (noun)	pine tree (noun)
low shrubby pine of central Europe with short bright green needles in bunches of two	a coniferous tree
tall pine of western North America with stout blue-green needles; bark is grey-brown with rectangular plates when mature
evergreen tree of New Zealand resembling the kawaka