Autumn has been stretched out this year with some of our trees only just preparing to drop their leaves as November marches towards winter.

This extended period has given a chance to see a difference between our native trees and some of the exotic species that grace our treescape, the range of colours on display. Most of our native trees fade from green to muted yellows and orange-browns before they lose their leaves but we have exotic species that put on a more spectacular display. There are the fiery reds of Japanese and Canadian Maples, the deep reds of American Oaks, and the deep purple of Liquidambar among others.

Autumn leaf fall with its associated colour changes is down to our temperate seasons. As winter approaches, the soil temperature and light levels drop. Tree roots find it more difficult to take up enough water and leaves produce less food for the tree while still needing water for transpiration. Most temperate broadleaved trees have to shed their leaves or die of thirst. Some such as Holly remain evergreen by having leaves adapted to lose much less water, usually by having a waxy coating.

Trees use a lot of resources to produce leaves, autumn colour changes are part of the trees’ efforts to recycle and reuse these resources. As I hope you know, leaves are green because they contain chlorophyll which is the main reagent in plants manufacturing their own food through photosynthesis. At the heart of each Chlorophyll molecule is Magnesium and the tree cannot afford to waste this metal which is not abundantly available in the soil. The answer is to take it back into the tree before the leaves drop. It is stored in the branches and trunk ready to be re-used in the spring.

Chlorophyll is only one of the pigments found in leaves. There are a range of yellow and orange carotenoids in leaves that are usually masked by the more intense green. These play a part in photosynthesis by absorbing some parts of the spectrum that chlorophyll misses. They do a number of other useful jobs in the life processes, one reason green vegetables are good for you. Carotenoids are complex molecules but are hydrocarbons, made only of carbon and hydrogen, the tree does not need scarce metals such as Magnesium to make them and so they stay in the leaf and just break down as the leaf decomposes after the chlorophyll is withdrawn. This is why so many autumn leaves turn yellow or orange-brown before they fall.

You might then ask about the more highly coloured species, the Maples and so on. Our autumn is characterised as the season of mists and mellow fruitfulness, we do not often enjoy prolonged strong autumn sunshine. In other climates such as parts of North America, China and Japan, autumn days are often very bright and there is enough sunshine to damage the leaves as they become paler while the chlorophyll is being withdrawn, quite literally they can be sunburned and killed before the tree will be able to gather in all the valuable Magnesium.

The answer that some trees have evolved is to manufacture a new set of pigments in the autumn, red and blue anthocyanins, that act as a temporary sun screen until the Magnesium is secured. The different combinations of reds and blues produce the dazzling range of colours seen in the New England Maple forests. These pigments are carbohydrates, compounds of carbon, hydrogen and oxygen, and can be dispensed with when the leaves fall.

Anthocyanins also act as indicators signalling ripeness in fruits. As with the carotenoids, these chemicals help in other life processes and red and purple fruits are an important part of a healthy diet for many animals. Some plants such Red Cabbage and Copper Beech have been selectively bred to have the Anthocyanins all year.

Whatever the time of year or colour, it is well worth taking a walk to enjoy Sidmouth’s exceptional treescape.