CEP- competitive exclusion principle. The CEP puts strict limits on the number of different species we should expect to find. The answer is very complicated, but essentially it comes down to the fact that competitive exclusion depends on a set of conditions that must be met before it can occur. Earth's environemnts would have to be an identical flat plain with no differences in temperature, moisture, etc. Droughts shift the competitive balance from species that need lots of water to those that don't, and back again.
Light becomes available at the surface again.
Light also means heat, which means evaporation. The environment has changed in a way that favors different species than before. Weeds shade the ground surface, making it slightly cooler, moister. This creates a microenvironment favorable to other organisms that aren't as tough as weeds, but that have their own advantages, such as shrubs, oak seedlings, pines, and others. As the oaks etc.