Trees, like most plants, use a green pigment called chlorophyll to photosynthesise – that is to produce sugars from the energy of the sun, using water and nutrients from the soil.
In autumn, trees that lose their leaves for winter go through a process to shut down photosynthesis and reclaim as many valuable nutrients as possible. Chlorophyll is constantly breaking down and being replaced through summer, but this process slows down in autumn. This reveals all those other chemicals in the leaves that were hidden by the presence of the dominant green chlorophyll.
These chemicals include yellow flavonols, orange carotenoids and red to purple anthocyanins. The exact mixture of these compounds varies between species, and hence the degree of yellow or red colour in the leaves.
Is it a protection from bright light? It is thought the red or purple anthocyanins, in particular, protect the leaf’s ability to photosynthesise by screening chlorophyll from bright light as the internal leaf structure begins to break down in autumn. The tree is allowed longer to reabsorb valuable nutrients as temperatures drop. (The brightest autumnal colours are found in North America where the coldest sunniest weather is found in autumn.)
As the tree becomes dormant, a compound called abscisic acid triggers a seal to develop at the base of the leaves, before they fall off. This reduces water reaching the leaf and traps the chemicals remaining in the leaves. The chemicals gradually break down, changing the colour of each leaf before it drops to the ground. As the process is gradual, individual leaves will be at different stages, so a tree will have leaves of many changing hues as autumn progresses.
The trigger for autumn colour? The trigger for autumn colour to develop is a combination of day length and night temperature. (In Britain today leaf fall is 10-18 days later than it was around 1940. I wonder why?) As days shorten, the amount of sugar generated by photosynthesis drops off, and hormones in the plant trigger the leaf sealing and shedding process. This is accelerated by cold nights.