Old growth forest.

This may be true, but it gets me thinking how would we really know this? Playing devil's advocate.

If the the natives that lived in the United States had engaged in practices that cut down a lot of forest in most regions or more likely burned a lot of forest in most regions, for sake of slash and burn agriculture or flushing game out, or just accidental forest fires from widespread use of fire, how would we know if a forest was 100 years old back then or a thousand years old back then.

Yeah they're going to be some places like where the giant sequoias and the giant Aspen rings were seen, that are going to be over a thousand years old, but it's not like there was a systematic survey of all those sites to indicate that it covered 50% of the country like this map shows. Even 5% would be enough to create plenty of records of old growth forest.

Maybe a lot of what we have left as old growth forest came out of a relatively small pool of old growth forest dating back that far. It's also possible that the 400 years of growth since the 1620, is enough to account for a decent amount of what we presently look at as old growth forest.