brushwood vs underwood

brushwood and underwood both are nouns.

Word	Noun	Adjective	Verb	Adverb
brushwood	Yes	No	No	No
underwood	Yes	No	No	No

As nouns, underwood is a hyponym of brushwood; that is, underwood is a word with a more specific, narrower meaning than brushwood:

brushwood: a dense growth of bushes
underwood: the brush (small trees and bushes and ferns etc.) growing beneath taller trees in a wood or forest

Other hyponyms of brushwood include brake, canebrake, spinney, underbrush, undergrowth.

brushwood (noun)	underwood (noun)
a dense growth of bushes	the brush (small trees and bushes and ferns etc.) growing beneath taller trees in a wood or forest
the wood from bushes or small branches