white aspen vs poplar tree

white aspen and poplar tree both are nouns.

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

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

white aspen: 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 white aspen include poplar.

white aspen (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