This happens through a process called decomposition, or rotting. The fungus and algae you see on the limb works with insects, bugs, bacteria and other animals to help this tree break down into soil. It will take many, many years. First you see fallen leaves and plants on the forest floor. The next layer includes bushes.
Younger trees grow up toward the top, ready to replace fallen trees.
The understory is made up of trees reaching toward the sun. Finally the canopy is formed by the highest trees. These trees get the most sun. The sun provides the energy for all the plants in the forest. The plants are primary producers in our great food web.