Changing colour for winter
14 November 2006
At this time of year, woodlands change colour and dazzle us with gold, copper or crimson foliage before the leaves fall. Ever wondered why?
The green pigments in leaves use sunlight to turn water and carbon dioxide into food. In the dark, cold months of winter, large, thin leaves are a liability. The green pigments are damaged by cold, and winter storms could break off branches and damage the tree.
The longer, cooler autumn nights trigger an amazing response. As the green pigments break down, the underlying yellow colour of the leaves shows through.
Trees grow scar tissue that gradually stops the flow of fluid to and from the leaves. Before their veins completely block up, the leaves try to pass as many nutrients to the tree as possible.
The bright autumn sunshine can damage the dying leaf before it has the chance to send all its goodness back, so to protect themselves some trees make a kind of sun-block. This red pigment - made from sugars in the leaf - buys time for the last of the food to be stored in the wood, ready for the spring.
Many different colours
Some trees are more colourful than others. Most British trees turn a variety of yellows and gold - including birch, horse chestnut and sycamore. A few, like beech, rowan and wild service tree, warm to a deeper shade of copper. It is left to native shrubs like dogwood and spindle, and introduced trees like maples, to turn a fiery red.
'over its lifetime, a birch or oak tree loses nearly three times as much weight in dead leaves! So, apart from blocking drains and gutters and slowing down trains, what use are they?'
Have you noticed that the leaves on the same tree may be quite different colours? If you look closely, you should see that the ones receiving the most sunshine are those that turn the warmest hues.
For the same reason, some years are more spectacular than others. Warm, sunny days and crisp, cool nights make for better autumn foliage, and drought years produce the most intense colours of all.
A heavy meal
You may think that wood is heavy, but over its lifetime, a birch or oak tree loses nearly three times as much weight in dead leaves! So, apart from blocking drains and gutters and slowing down trains, what use are they?
If you look on the floor of a deciduous woodland, you will see that the litter layer is quite thin - usually only a few centimetres deep. This is because, once dead leaves have been conditioned by the weather, they form a rich source of food for a whole range of invertebrates and micro-organisms.
Munching the mulch are mini-beasts such as earthworms, beetles, mites and spring-tails. With the help of bacteria and fungi, these small creatures play an important role in returning minerals to the soil, so that trees can make use of them again.
It can take many weeks for rain to make the leaves edible. In the meantime, drifts of dry leaves are used by some of our favourite animals to line their burrows or nests, or to keep them snug and warm through cold winter nights. So, think twice before you burn that pile of dry leaves - it may be home to queen bumblebees, slow worms, newts or hibernating hedgehogs.
It’s mulch better than peat!
Rotted leaves make a very good peat alternative. To do this, pile them up in a fenced-off part of the garden to prevent them blowing around, and turn them occasionally (watch out for sleeping tenants - see above). The mini-beasts get to work, producing an excellent leaf mould.
Don't add the the leaves to your compost heap though, because it makes the heap cooler and slows down the rotting process. In 1-2 years, you will have lots of leaf mould, which you can use as a mulch, dig in, or mix with compost for potting up plants or growing seeds.