Why do leaves change colour in Autumn?

Autumn offers us a burst of colour and beauty in our gardens before the Winter sets in. But what really makes the leaves on the trees turn from green to colours such as red / brown and yellow?

Tress normally change the colours of their leaves due to changes in the way in which they obtain their food. Trees, like a majority of our garden plants, produce food by the process known as Photosynthesis?

Photosynthesis is a process used by plants and other organisms to convert light energy, normally from the sun into chemical energy that can be used to fuel the organism's activities. Source WikiPedia

• By product of Photosynthesis is Oxygen.
• Sunlight energy is captured by chlorophyll (green chemical pigment found in leaves).
• Converts carbon dioxide and water into organic sugars.
• The sugars are stored in leaves or other parts of the plant for growth.
• By product of Photosynthesis is oxygen.

Now we know that leaves are green due to containing chlorophyll, we will understand that during autumn and early winter when there is less sun around leaves start to drain of this 'plant food' which causes decomposition causing the green to fade.

Surging sugar concentrations cause increased production of anthocyanin pigments. Leaves which contain  primarily anthocyanins will appear red. Another pigment class is carotenoids which produce orange, yellow or red. Anthocyanin and carotenoids will appear orange. Tannins are responsible for the brownish color of some oak trees.

Factors effecting chemical reaction of leaves:

Cold temperature: Low temperatures destroy chlorophyll causes green to fade to yellow. If temperatures stay above freezing then anthocyanin production is enhanced and the leaves take on a red colour.

Dry weather: Causes sugars to become concentrated in leaves which produces more anthocyanin production.

Sunny days: Chlorophyll stops in the autumn though photosynthesis can still occur on sunny days which uses the remaining chlorophyll. Sugar concentration thus increases producing higher anthocyanin percentages and the leaves turn to red.

To see the best in colours of autumn leaves then we should of had a warm and dry summer with an autumn of cold, but not freezing days and fairly dry which will intensify the colours.