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. Hemlock seedlings need cool, moist conditions created by the oaks. They sprout and grow up under the oaks. Without further disturbance, the forest may maintain itself as a hemlock forest.