western larch vs larch tree

western larch and larch tree both are nouns.

Word	Noun	Adjective	Verb	Adverb
western larch	Yes	No	No	No
larch tree	Yes	No	No	No

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

western larch: tall larch of western North America have pale green sharply pointed leaves and oblong cones; an important timber tree
larch tree: any of numerous conifers of the genus Larix all having deciduous needlelike leaves

Other hypernyms of western larch include larch.

western larch (noun)	larch tree (noun)
tall larch of western North America have pale green sharply pointed leaves and oblong cones; an important timber tree	any of numerous conifers of the genus Larix all having deciduous needlelike leaves