Revealing the colours of autumn

A maple at Knowle.

A maple at Knowle. - Credit: Archant

Ed Dolphin, treasurer of Sidmouth Arboretum and newly appointed Sid Valley Tree Warden takes a look at the autumn colours.

Mahonia at Knowle.

Mahonia at Knowle. - Credit: Archant

Like a glorious sunset before the long night of winter, the valley’s trees are putting on their defiant display of autumn colour.

Trees and shrubs come in a wide array of greens during the spring and summer because the leaves contain chlorophyll.

The leaves are the food factories of the plants where the chlorophyll traps the sun’s energy to power sugar production.

In most plants, including trees and shrubs, the green colour masks the presence of other coloured chemicals, yellow flavonols, orange carotenoids and anthocyanins that range from red to purple, these play their own part in the leaves’ biochemistry.

Autumn colour at Bicton

Autumn colour at Bicton - Credit: Archant

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All this activity is dependent on the leaves having enough water.

Our trees have had a tough summer with water in short supply, but most have survived because they have deep roots to access water. It might seem odd but trees struggle to obtain enough water in winter. The ground might hold more water, but the cold stops the roots working and the trees cannot access it.

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Evergreen trees are adapted to cope with this, either they have reduced leaves like needles or scales or the leaves have waxy coatings to reduce water loss. Deciduous trees have to shed their leaves in winter because they do not have these adaptations and their leaves lose too much water.

As the autumn days shorten and the soil temperature drops, deciduous trees begin to break down and withdraw the valuable goodness from the leaves. Chlorophyll is one of the first resources taken back and this reveals the other colours.

Autumn colours at Knowle.

Autumn colours at Knowle. - Credit: Archant

The actual shades that we see will depend on the mix of chemicals. Ginkgos carry a lot of flavanols and they turn bright yellow. Japanese Acers actually produce new red anthocyanin as the chlorophyll breaks down. This protects the remaining tissues from damage by the light energy that the chlorophyll is no longer absorbing, this allows the tree more time to reabsorb other useful resources before the leaf dies.

The final act of self-preservation is for the tree to shed its leaves. An abscission layer of cork grows across the petiole to deny water and kill off the leaf completely. The remaining tissue of the leaf dies and turns brown, this is caused by oxidised phenolic chemicals, much the same as an apple turning brown if you bite into it and leave it. Finally, the leaf falls off, the scar is sealed by the cork.

We get the best colour display if we have a warm dry September allowing the colours to develop, followed by a quick drop in temperature with colder nights in October. In a mild wet autumn, the whole process is much slower and many leaves are completely brown before they fall which reduces the whole spectacle.

To find out more about the trees of the Sid Valley, visit the Sidmouth Arboretum website

A gingko at Knowle.

A gingko at Knowle. - Credit: Archant

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