yellow poplar vs yellow poplar

yellow poplar and yellow poplar both are nouns.

Word	Noun	Adjective	Verb	Adverb
yellow poplar	Yes	No	No	No
yellow poplar	Yes	No	No	No

yellow poplar (noun)	yellow poplar (noun)
tall North American deciduous timber tree having large tulip-shaped greenish yellow flowers and conelike fruit; yields soft white woods used especially for cabinet work	tall North American deciduous timber tree having large tulip-shaped greenish yellow flowers and conelike fruit; yields soft white woods used especially for cabinet work
light easily worked wood of a tulip tree; used for furniture and veneer	light easily worked wood of a tulip tree; used for furniture and veneer