it's theoretically possible to "put it all back", i.e. get the percentage of forest land back to what is was in 1600.
What needs to be remembered (as I tried to point out) is that when settlers arrived from Europe they were pleasantly welcomed with already cleared forests, or at the very least, rather young growth. Native Americans were much, much more developed than what most school books teach, and to support their millions with what would have been low yield farming would have required huge areas of farming.
However, European diseases swept through well before most settlers set foot in an area, and Indian villages which probably had thousands of inhabitants were reduced to hundreds or even just a few dozen, or were completely abandoned all together. The fields, already flattened and cleared for centuries, were left to go fallow as nobody was there to tend them, and when settlers came they had little trouble bringing them back to life (so no, there wasn't nearly as much 'old growth' forest as imagined pre-1600).
This is also true in the Amazon (still a theory, but becoming generally accepted). The massive rainforest that many say has been there since forever probably wasn't as big as today, and the indication for this is the soil. Called "terra preta", this super rich dirt found all over the Amazon (in what is now dense jungle) was actually
man-made. Unbelievably huge swaths of it. Its not natural and probably covered an exponentially larger area at the time (its still covers about 10% of the Amazon basin ...about the size of Montana). And considering the cities and giant population that existed up to when Europeans arrived, it'd be understandable that such large amounts of farmland would be necessary (again, especially with low yield farming). But then again, considering that Native Americans were living in these places for over 15,000 years, one would imagine they would have altered the landscape to a large degree.
