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forest vs old growth old growth vs forest

forest and old growth both are nouns.

forest is a verb but old growth is not a verb.

As nouns, old growth is a hyponym of forest; that is, old growth is a word with a more specific, narrower meaning than forest and old growth is a type of forest with the definitions:
  • the trees and other plants in a large densely wooded area
forest (noun) old growth (noun)
the trees and other plants in a large densely wooded area forest or woodland having a mature or overly mature ecosystem more or less uninfluenced by human activity
land that is covered with trees and shrubs
forest (verb) old growth (verb)
establish a forest on previously unforested land