Jobs of a living leaf:
While attached to a tree, leaves perform three important tasks:
1. They are the ‘food factory’ for the tree. Photosynthesis happens in the leaves.
2. They provide food for wildlife. The larvae of many insects, including moths and butterflies, feed on tree leaves.
3. They provide shelter for wildlife. The most obvious might be the nest hidden away in a secret spot, but leaves also provide shelter for birds and insects during inclement weather.
Jobs of a fallen leaf:
Fallen leaves actually do the same jobs, providing shelter and food, though in quite different ways than when they were part of the tree.
1. Layers of fallen leaves provide shelter for wildlife. Peek under a pile of leaves and you will find a cool, moist, temperature-controlled environment perfect for creatures like worms, beetles, springtails and centipedes, plus many animals too small to be seen with the naked eye. Many of these animals feed on decaying organic material (like dead plants and animals). However, old leaves also provide temporary shelter for overwintering animals, like bumblebees and butterfly larvae.
2. Leaf litter provides food for decomposers. When a leaf falls to the ground, the first player in the decomposition process is water. Water softens the material and leaches out sugars and other compounds. The detritivores (litter-feeding invertebrates) take over and begin breaking down the leaves further by nibbling. Detritivores include larger creatures like earthworms, woodlice and millipedes, smaller animals (barely visible to the naked eye) like springtails and mites, and microbes (not visible to the naked eye). Leaf litter is also food for fungi, which are the primary decomposers of plant material. Of course, where there are plant eaters, there will also be predators, like spiders, centipedes, beetles, bats, birds, lizards... and rats. Once the decomposition process is complete, nutrients locked up in the leaves are once again available in the soil to be used by the ‘parent’ tree.
(Information from: http://www.kidsdiscover.com/teacherresources/new-life-old-leaves/)
In the beech forest, the layer of leaf litter is rather thin because beeches are evergreen, yet very important to the functioning of the beech forest ecosystem.