Why do leaves change colour in autumn?

Every autumn we revel in the beauty of leaf colours. The mixture of red, purples orange and yellow is the result of a chemical process that takes place in the tree as the seasons change from summer to winter.

Here comes the science bit

During spring and summer the leaves have served as factories where the food necessary for growth are manufactured. This process takes place in the leaf cells containing chlorophyll which gives the leaf its green colour. This chemical absorbs from sunlight the energy that is used to transform carbon dioxide and
water to carbohydrates, such as sugar and starch. Along with the green pigment are yellow and orange pigments which are masked by large amounts of green colouring.
 

Awesome autumn

In autumn triggered by shorter days and lower temperatures the leaves stop their food making process. The chlorophyll breaks down, the green colour disappears and the yellow and orange colours become visible. At the same time other chemical changes may occur which form additional colours through the
development of red pigments. Some mixtures give rise to reddish and purplish autumn colours such as dogwoods and sumachs, while others give maple its brilliant orange. The autumn foliage of some
trees show only yellow colours, whilst others like oaks show only brown. All these colours are the varying amounts of the chlorophyll residue and other pigments in the leaf.
 

What effect does climate have?

Temperature, light and water supply have an influence on the degree and duration of leaf fall. Low  temperatures above freezing will favour red colour formation. An early frost will weaken the red colour. The brilliant autumn foliage colours of New England are the result of bright, sunny autumn days combined with cool nights acting on species such as maples, dogwoods and oaks. The more muted autumn colours in Britain reflect our cooler, damper and more overcast conditions.