Leaves Leaves Everywhere
Trees which shed all their leaves for part of the year are known as deciduous (or broad leaf) trees. These trees shed their leaves in order to conserve water and energy over the winter months. Hormones within the tree trigger the process of abscission, which causes the tree to lose its leaves. Abscission comes from the latin ab, meaning away, and scindere meaning to cut.
At the start of abscission, trees reabsorb valuable nutrients from the leaves and store them for later use in their roots. Chlorophyll (which gives leaves their green colour) is one of the molecules that is broken down for its nutrients and this is one of the reasons why leaves change colour in Autumn.
In the natural environment, leaf litter is a key part of the nutrient cycle, as it allows nutrients which remain in the leaves when they fall, to be returned to the soil through decomposition. This is also true in the garden environment. However, for aesthetic purposes, the leaves are often collected and stored in leaf heaps, before the resulting leaf mould is returned to the soil the following year.