underbrush vs forest

underbrush and forest both are nouns.

Word	Noun	Adjective	Verb	Adverb
underbrush	Yes	No	No	No
forest	Yes	No	Yes	No

underbrush (noun)	forest (noun)
the brush (small trees and bushes and ferns etc.) growing beneath taller trees in a wood or forest	the trees and other plants in a large densely wooded area
	land that is covered with trees and shrubs

underbrush (verb)	forest (verb)
	establish a forest on previously unforested land