Again, that's a very good question.
One of the fundamental tenets of third party certification is that a third party auditor comes in to validate that you are harvesting with the most benign or least damaging footprint, protecting the streams and the contours of the land. Your reforestation practices come in right after that. You must maintain those for at least a period of five years until, as the foresters will tell you, they are free to grow. You don't just throw on a bunch of seeds and walk away. You actually have to maintain it until it's in a growing state, and that's part of the certification process. It is not just that the companies do it; they are audited by third party auditors to make sure they're doing it.
It's in the interests of the companies to make sure they have product in years to come.