The landscapes are undergoing desertification. Where 200 years ago it may have been a prime environment to grow a new forest, and an existing forest could endur

The landscapes are undergoing desertification. Where 200 years ago it may have been a prime environment to grow a new forest, and an existing forest could endure, growing a new forest is not likely to succeed due to temperature/water availability. Under your given time span of “hundreds of millions of years” I assume essentially square inch of land underwent desertification at some point, if it wasn’t buried beneath the sea. I mean, there weren’t even trees before 385 million years ago.