old growth vs forest

old growth and forest both are nouns.

Word	Noun	Adjective	Verb	Adverb
old growth	Yes	No	No	No
forest	Yes	No	Yes	No

As nouns, forest is a hypernym of old growth; that is, forest is a word with a broader meaning than old growth:

old growth: forest or woodland having a mature or overly mature ecosystem more or less uninfluenced by human activity
forest: the trees and other plants in a large densely wooded area

Other hypernyms of old growth include wood, woods.

old growth (noun)	forest (noun)
forest or woodland having a mature or overly mature ecosystem more or less uninfluenced by human activity	the trees and other plants in a large densely wooded area
	land that is covered with trees and shrubs

old growth (verb)	forest (verb)
	establish a forest on previously unforested land