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Autumn Colours

Looking up at autumn trees. Photo Chris MaguireLooking up at autumn trees. Photo Chris Maguire

By Barbara Thompson

Have you been out to see the leaves this year?

Every autumn the countryside is cloaked with this fluttering coat of many colours - red, yellow, green, black, purple, orange, violet, gold - every hue and tint you can think of is represented in the blaze of glory that trumpets out the dying year.

It seems like it’s a show put on for our benefit alone – what does it matter to the tree whether it drops a cascade of molton gold or a sad sack of brown withered peelings? But these cast offs are important for the tree as they decay down to leaf mould and merge into the black earth again ready to be used by the tree once more.

For trees, growing leaves and then losing them is a complex process, part of the annual cycle of growth. In summer they are food-producing factories. They are green because of the chemical chlorophyll, which in a fiendishly complicated chemical process, uses sunlight to turn water from the roots and carbon dioxide from the air into oxygen and sugars.

As the days become shorter and the sunlight decreases deciduous trees react by starting to grow a corky layer of cells between the twig and the leaf, cutting off the flow of supplies in both directions. With fewer sunny hours the chlorophyll also breaks down quite quickly and reveals that it has been masking other vibrant chemical pigments in the leaves. If the pigment carotenoid (as in carrots) is present the leaves will turn red and orange, with xanthophyll yellow predominates, tannins give browns and anthocyanins can produce red, blue or purple, depending on the acidity or alkalinity of the soil. These last are responsible for the stunning scarlet of the North American maples when a last surge of sugar from the tree stimulates a firework burst of anthocyanins. Just like us the tired leaf appreciates a spoonful of maple syrup!

Here in Britain the colours are more subtle with butter-yellow ash and birch, ochre beech, cream and purple elder and toast-brown oak. But if the autumn continues without too much wind or torrential rain some may last well into November. Autumn leaves on the trees or the ground, they’re worth a weekend walk to find. Try Bowness-on-Solway or Wreay Woods Nature Reserves.  

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