The process by which trees lose structures, such as leaves, is called abcission. Deciduous trees are those that lose all their leaves for a period of the year, which many spend dormant. Typically, this time of year is cool to cold, wintertime for most. The leaves are no longer needed for this period of dormancy, so the tree secretes a chemical which causes the leaf to cease it's photosynthetic properties, largely due to the loss of the chlorophyll in the chloroplasts of the leaves. This is when the leaves turn a variety of other colors. At some opportune moment, the leaf is abcised from it's perch on the tree limb and it flutters away to the ground, to decompose into rich, decaying organic matter. It is at this point the leaf may be referred to as a "dead leaf".