Autumn Colours Explained
"Different trees, even different parts of the same tree, pass through these changes at slightly differing rates."
There’s something about trees that speaks to our soul. They’re solid and dependable, yet never the same. Their swaying, rain-slick branches broadcast the weather better than any presenter. Their dynamic colours are a reliable barometer of the seasons, inspiring us annually with their autumn display. But what causes their extraordinary transformation at this time of year? What’s going on and how do they know when it’s time to turn?
Autumn is when our native British trees come into their own in the Oxfordshire countryside. On a bright October day at the National Trust’s Badbury Hill in West Oxfordshire, on the wooded estate at Greys Court or on the steep edge of the Chilterns at Watlington Hill, our rolling woodlands are aflame with colour and it’s the perfect time for a walk. National Trust forester, Tony Knight knows better than most the power of an autumn woodland. He’s worked on the conservation charity’s Coleshill Estate in Oxfordshire since he left school, following in the footsteps of both his father and grandfather who worked as foresters in the very same woodlands before him.
‘I often find myself working in plantations that my grandfather planted,’ says Tony. ‘I feel like they’re my woods now. I played here with my brothers and friends when I was a child, worked here in school holidays with my dad, and now I’m the one looking after them.’
Tony firmly believes that autumn colour in the woodland starts with the kind of summer we have. He’s generally working on the estate’s grassland in summer, but in the woodlands, the leaves are busy soaking up the sunlight of the long days for the tree to use as fuel. The green pigment (chlorophyll) absorbs the light and uses it to process carbon dioxide from the air and water from rainfall to create oxygen and glucose. Through this process of photosynthesis, the sugar is used as energy to grow the tree and the oxygen diffuses into the air that we breathe.
‘After a dry summer with lots of sunshine, autumn comes early and as long as there’s no frost, you’ll get good autumn colour that lasts,’ says Tony. What’s happening after a good summer is that the trees have been able to build up plenty of sugars in their leaves. But as the daylight hours get shorter towards autumn, the sun is at a lower angle and the air becomes cooler which triggers changes in the plant chemistry of deciduous trees. The veins that carry fluids in and out of the leaf gradually close off, the chlorophyll breaks down and the strong green pigments fade away. The yellow and orange pigments, which have been there all along, but obscured by the bright green, are freed to flame into colour.
‘Some of the best trees for rich red and purple colours are maples, oaks, cherries and dogwoods,’ says Tony. These red and purple shades appear when glucose is trapped in the leaves when photosynthesis stops. Under autumn sunlight and cool nights, the glucose is chemically broken down, causing those bright crimson and plum colours. The brighter the sunlight in the autumn, the greater the production of anthocyanin (red and purple) pigments and the more brilliant the colour display.
Different trees, even different parts of the same tree, pass through these changes at slightly differing rates. This gives us the dynamic, shifting range of autumnal colours we enjoy each year. Autumn can come as early as mid-September but if it stays dry, the colours can continue well into November – although a severe frost or storm can bring the display to an abrupt end.
Autumn and winter are Tony’s favourite time of year. It’s when he gets to return to the woodland at Badbury Clump or Flamborough Wood to thin out trees that have been planted too close together and give them room to breathe. He’ll also plant new trees, or coppice the hazel. Hazel coppicing is a traditional way to look after this type of tree. Cutting them back hard to a foot or so off the ground helps them to regenerate, rather than get leggy and die off early.
At the end of the autumn, the redundant leaves are finally shed from the trees to form leaf litter. This breaks down into the soil to provide humus and plant nutrients for future years, and food for soil organisms, such as worms and fungi.
During winter, there is insufficient light and warmth for photosynthesis and in icy conditions water may be in short supply. In response, deciduous trees become almost dormant, using the energy they stored during the summer to create new leaf buds in the following spring.
It’s a circle of nature that never grows old for Tony Knight and his family. He has no intention of giving up his work in the woods on the Coleshill Estate. ‘They’ll have to carry me away in a box from here,’ he jokes. There’s a fourth generation of Knights with shining chainsaws ready to take up the mantle as Tony’s son also works on the estate as a forester. Autumn will continue to flame bright in healthy woodlands around Coleshill.
For a great range of autumn walks with step-by-step directions around the Coleshill Estate, head to the Coleshill website.
Images © National Trust / Hugh Mothersole
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