Autumn colour is a natural wonder (Winkworth Arboretum)
In countries that have an autumn season, when the weather gets cooler and the days get shorter, the leaves of deciduous trees change from green to shades of yellow, orange and red before falling away completely. It can make a spectacular display, but we know it’s not all a show for us: so what is really happening and why are the trees doing this?
Deciduous trees have evolved to cope with the damage that the freezing weather of winter brings by shedding their soft, vulnerable leaves. The leaves are where the chlorophyll is found, the chemical that is needed by the tree to use the sun’s energy to convert water and carbon dioxide into sugars – the form of energy that can be used by the tree. Chlorophyll is a green pigment, which is why leaves appear green during the spring and summer.
However, as days shorten in autumn, the process of leaf senescence begins, which eventually leads to leaf fall. As part of the process the tree begins to remove as much of the nutrients as it can from the leaves and stores them within the woody tissue, until they can be reused in the spring to produce new leaves. The chemicals within the leaf, including carbohydrates and proteins, are broken down into smaller units for storage. Chlorophyll is one of the chemicals broken down and as it is withdrawn from the leaves and stored in the body of the tree, its green colour is lost from the leaf, so that it then takes on the colour of some of the other chemicals that are there.
Yellow leaves are most common in European species
The commonest colour of autumn leaves (in European species, at least) is yellow, sometimes veering towards orange. This is caused by carotenoids, which are contained in all leaves as it provides protection from the damaging effects of sunlight. Trees need sunlight to photosynthesise, but paradoxically, too much sun is harmful. If the summer has been very sunny, the autumn leaves will be a brighter yellow.
Red leaves are brightest after a cold snap
Some trees (such as many maples) have red autumn leaves, which is caused by anthocyanins. These pigments are produced in some trees as the chlorophyll is withdrawn, and scientists aren’t entirely certain why. Anthocyanin production seems to be higher where colder weather prevents sugars being withdrawn from the leaves so quickly. The anthocyanins may help protect the leaves from sun damage, giving the leaf more time to withdraw nutrients into the tree. For us, it means that a cold snap will produce redder leaves.
The variety of colours seen in autumn leaves are due to genetic differences of tree species and varieties and also to the subtle variations of weather. Periods of drought and wet, sun and cold at different times during the previous season make some autumn displays spectacular and others more muted. Some leaves may even start to turn brown while still on the tree, due to the pigments starting to decay.
Fallen leaves decay and recycle nutrients
Finally, as the tree completes the process of recovering nutrients from the tree, a corky layer of cells called the abscission layer is formed at the base of the leaf. This eventually seals off the leaf from the tree, and it will fall to the ground – how quickly depends on the strength of the wind. Once on the ground the leaves gradually decompose, releasing even more nutrients into the soil where the tree can recover them through its roots (although this is a much slower process). The tree then remains dormant until the spring, when the length of the day increases again, temperatures climb, and all those nutrients are needed once more to produce new leaves and continue the cycle.