bitter-bark vs shrub

bitter-bark and shrub both are nouns.

Word	Noun	Adjective	Verb	Adverb
bitter-bark	Yes	No	No	No
shrub	Yes	No	No	No

As nouns, shrub is a hypernym of bitter-bark; that is, shrub is a word with a broader meaning than bitter-bark:

bitter-bark: ornamental shrub or small tree of swampy areas in southwestern United States having large pink or white sepals and yielding Georgia bark for treating fever
shrub: a low woody perennial plant usually having several major stems

Other hypernyms of bitter-bark include bush.

bitter-bark (noun)	shrub (noun)
ornamental shrub or small tree of swampy areas in southwestern United States having large pink or white sepals and yielding Georgia bark for treating fever	a low woody perennial plant usually having several major stems