Chilterns Countryside’s autumn colours explained
One of the great pleasures in the Chilterns Countryside is a bright autumn day when the leaves are turning from their summer green to a myriad of shades of yellow, gold, orange, red and brown.
Summer’s green leaves
The green shades of summer leaves are a result of chlorophyll; a group of closely related green pigments found in the leaves of plants, which is essential for the production of sugars, especially glucose. Chlorophyll absorbs light, mostly in the blue and to a lesser extent red portions of the electromagnetic spectrum, hence its intense green colour. The light energy is then used to combine carbon dioxide from the air and water from rainfall, which reaches the leaves via the plant’s roots, into oxygen and glucose. This process is called photosynthesis. Plants use glucose as a source of energy and as a building block for growth. The oxygen is released into the air and it is breathed by us and all other animals.
Fading sunlight and cooling air
As summer ends, the daylight hours get shorter, the sun crosses the sky at a lower angle, and the air becomes cooler. Under these conditions, there is insufficient light and warmth for photosynthesis and in icy conditions water may be in short supply, so deciduous trees shed their leaves so they can conserve moisture over the winter months. As the trees begin to shut down in the autumn, the veins that carry fluids into and out of the leaf are gradually closed off, eventually leading to a process known as abscission.
Firstly, each tree reabsorbs many of the nutrients from the chlorophyll as it degrades, then a protective layer is formed where the leaf stems meet the tree, and finally the leaves becomes detached and are blown off by the wind or fall under their own weight. The tree survives through the winter on the sugars it has stored during the summer and it uses this stored energy to grow new leaf buds the following spring.
Yellows and oranges
As the green chlorophyll breaks down and the strong green pigments fade away, we begin to see yellow and orange colours (carotenoid pigments). These colours have been in the leaves throughout the summer, but hitherto they have been obscured by the bright green chlorophyll.
To see some of the best autumn colours in the Chilterns Countryside, visit one of our beech woodlands at Aston Wood, Bradenham, Greenfields Copse, Juniper Bank, Pulpit Hill or at Low Scrubs near Coombe Hill.
Reds and purples
In some trees, like maples, oaks, cherries and dogwoods, glucose is trapped in the leaves when photosynthesis stops. Under autumn sunlight and cool nights, the glucose is chemically broken down, leading to shades of red or purple (anthocyanin pigments). The brighter the sunlight during this period, the greater the production of anthocyanins and the more brilliant the resulting colour display.
Look out for brightly coloured dogwood at Watlington Hill and Coombe Hill. Wild cherries and oaks can be found in all our woodlands, especially at Bradenham and Pulpit Hill. The guelder roses at Grangelands near Pulpit Hill have both red foliage and red berries in the early autumn.
Different trees, even different parts of the same trees, pass through these changes at slightly differing rates, which leads to the beautiful range of autumnal colours we enjoy each year. If the spring and summer are warm and wet, with above average sunshine, good growing conditions prevail and the trees build up plenty of sugars in their leaves. This can lead to the richest autumn colours.
“All the leaves are brown”
In many tree species, just before they are shed by the trees, the leaves will finally turn brown: the colour of the dead and decaying cells and any waste products remaining in the leaves.
At the end of the autumn, the redundant leaves are finally shed to form leaf litter, which eventually, breaks down into the soil to provide humus and plant nutrients for future years, and food for soil organisms, such as worms and fungi.
When are the autumn colours at their best?
Autumn can come as early as mid-September but, continued dry conditions and the absence of early frosts can mean the colours may not be at their most vibrant until mid- to late-October. A mild autumn could lead to prolonged colours well into November, although a severe frost or a lively autumnal storm can bring the colourful display to an abrupt end.
For the best way to see autumn colours, we recommend following one of our Chilterns Countryside autumn walks.