balsam poplar vs poplar tree

balsam poplar and poplar tree both are nouns.

Word	Noun	Adjective	Verb	Adverb
balsam poplar	Yes	No	No	No
poplar tree	Yes	No	No	No

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

balsam poplar: poplar of northeastern North America with broad heart-shaped leaves
poplar tree: any of numerous trees of north temperate regions having light soft wood and flowers borne in catkins

Other hypernyms of balsam poplar include poplar.

balsam poplar (noun)	poplar tree (noun)
poplar of northeastern North America with broad heart-shaped leaves	any of numerous trees of north temperate regions having light soft wood and flowers borne in catkins