Autumn Leaves 2016

Poplar Leaves - 2 November 2016
When on the tree, the yellow and red leaves would have been admired;
on the ground, the range of colours is actually increased.

We all admire the brilliantly coloured leaves of autumn while they are still on the tree. But when they fall off the tree, onto the ground, our attitude changes; their beauty is forgotten and we see them only as a nuisance. They make our streets and gardens untidy; they block the drains; and, they form a slippery layer on the railway lines, stopping the trains running to time. All that is left is to collect them up, and either burn them or put them in the rubbish; except for gardeners, who use the dead leaves for compost.

Contrasting Leaves - 8 November 2016
The maple leaf contrasts strongly with the surrounding oak leaves
in colour, shape and texture.

But there is still plenty of visual interest in the leaves as they rot away on the ground.  While the vibrant colours remain for a few days at least after the leaves have fallen, the range of colours is increased, not only by browns, dark purples, and blacks of the decomposing leaves, but also by different coloured leaves from adjacent trees.  Similarly, there are strong contrasts in shape and texture when a leaf from one tree falls on the leaves from a different type of tree.

Why Leaves Turn Red and Yellow

Fallen Leaves - 8 November 2016
Most of the colour of dead leaves derives from the breakdown of chlorophyll
unmasking yellow and orange pigments already present.

The process by which a living green leaf, producing sugar and oxygen from sunlight and air, turns bright red or yellow, and eventually falls to earth and rots away, starts when the length of the night exceeds a certain value. The tree then builds a barrier between the leaf and the rest of the tree, cutting the leaf off from nutrients absorbed through the roots, and tree from the sugar created in the leaf. As a result the green chlorophyll pigment decays, and exposes the yellow and orange colours of the xanthophylls and carotenoids normally present in the leaf. 
 

Maple Leaves - 8 November 2016
Red pigmentation develops after the leaf is already dying.
What is the advantage to the tree?

Interestingly, the red colours come from a third group of pigments, the anthocyanins, which are only synthesised when the leaf is dying. It is not entirely clear what the advantage to the tree is to have red leaves in autumn. Theories include: to warn off aphids which might want to use the tree as an overwinter host; to undermine the camouflage of herbivores; or, to attract birds to the tree to eat berries which may otherwise be overlooked. The anthocyanins in maple leaves have been shown to stunt the growth of any nearby saplings.

The Weather For It

The best display of autumn colour is produced when a moist growing season is followed by a dry autumn with sunny days and cool, but not frosty nights. Heavy rain, gales and frost are all likely to bring a premature end to the display.

Folklore, Legend and Medicine

Oak Leaves - 8 November 2016
With a mass of leaves like this, it is easy to see why the Lakota believed
they had been dropped as a protective blanket.

Fallen leaves, though such a prominent feature of the countryside in their season, do not feature greatly in folklore. They have been associated with fairies, with a swirl of leaves believed to be evidence of fairies dancing. Catching a falling leaf, particularly at the beginning of autumn, is said to protect the catcher from illness, or, more specifically, colds, throughout the winter.

But, perhaps, the most charming legend is that of the Lakota American Indians. The god who looks after living creatures saw the suffering of the plants and flowers as they shivered in the increasing cold of Autumn. Feeling sorry for them, the deity ordained that the trees should shed their leaves over the earth as a blanket to keep the plants warm. In return, the trees were allowed one great last blaze of glory.

I could find no reference to any medicinal use of fallen leaves.  However,  a Finnish company is developing the technology to extract natural pigments from the leaves for use in the clothing and cosmetic industries. The residual biomass is high in nutrients with possible uses as a fertiliser.

Next: More Autumn