Trees grow back faster than you might think. Especially the species and grades that go into papermaking. Nobody is cutting down old-growth forest and putting it into pulp. Unless they are a complete idiot - there's no money in it; it's been a while since I was doing my father's logging invoices, but his margins on spruce and fir or hardwood pulp were razor thin, after accounting to stumpage to the landowner and operating expenses. It was largely a way to get rid of the stuff that needed to be cut, but that he couldn't convince anyone to scale out for veneer or saw logs. A huge part of the pulpwood was also blowdowns, storm damage, and disease or insect culls.