Leaves change colour in response to shortening days and cooler temperatures. Chlorophyll production reduces, resulting in diminishing photosynthesis and revealing other pigments such as Carotenoids and Anthocyanins. These are responsible for our rich autumn yellows, oranges and purples. This process, called senescence, eventually causes the leaves to drop. Trees can turn early in the season if they are under stress, if they have dried out during summer or the earth is compacted around the roots. A good layer of mulch can help them, but this should be kept clear from the base of the trunk.
Fallen leaves if collected and heaped into a pile not only create a mulch that can be used after it has broken down (this takes a couple of years), but also creates a great habitat all kinds of creatures such as frogs, toads and hedgehogs.
Trees and shrubs can create good autumn/winter colour. Beech trees in their natural setting become large but, if regularly clipped, can create a dense hedge that holds onto its leaves during winter. This is great for nesting birds in early spring. Dogwood (Cornus) is a shrub giving red stems when all the leaves are lost. The richest colour comes if it is pruned down to the ground in the spring. Japanese Maple turn a fantastic deep red and also creates a red carpet beneath the tree as the leaves fall. These are slow growing, so good for the urban garden or in patio pots.