Very little sunlight reaches the ground in a mature stand of trees, which means that grasses, wild raspberries, and other sun-loving plants cannot grow here. As these plants disappear, so do animals like rodents and rabbits that depend on them for food. Animals that eat rodents and rabbits aren't far behind. Sunlight also affects the types of trees that grow to replace older ones. Some types of tree seedlings need very little sunlight to grow.
Maples are a good example.
They can grow in the deep shade of a mature forest. Oaks, on the other hand, need a good supply of sunlight to get started. Frequent natural wildfires used to thin out forests and favor the growth of sun-loving species. During the past century, we've done such a good job of controlling wildfires that many of our oak-hickory forests are being taken over by maples. While maples are popular with people, they aren't nearly as valuable for wildlife as oaks, hickories, and other nut-bearing trees.