aspen poplar vs poplar tree

aspen poplar and poplar tree both are nouns.

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

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

aspen poplar: a poplar that is widely cultivated in the United States; has white bark and leaves with whitish undersurfaces
poplar tree: any of numerous trees of north temperate regions having light soft wood and flowers borne in catkins

Other hypernyms of aspen poplar include poplar.

aspen poplar (noun)	poplar tree (noun)
a poplar that is widely cultivated in the United States; has white bark and leaves with whitish undersurfaces	any of numerous trees of north temperate regions having light soft wood and flowers borne in catkins