Leaf-fall

Image: Lukas Riebling

All but one of our native British trees are deciduous, shedding in a few weeks of autumn their canopy of leaves. The leaves contain green chlorophyll, held in bodies known as chloroplasts. It is in the chloroplasts that the main business of the tree is carried out – photosynthesis, the manufacture of sugars and other organic compounds, using carbon dioxide, water and trace elements, the whole reaction being powered by sunlight.

As much attention to detail is lavished on an individual leaf as on the whole tree. Each one is a miracle, a factory filled with chloroplasts. To work properly the chloroplasts must be kept wet, so the leaf is enveloped in a waterproof envelope and air is only allowed to enter through special pores which can be opened or closed at will.

Then of course the products of the chloroplasts must be conveyed to the rest of the tree, and waste matter taken away, so the leaf is provided with a complete network of veins.

When you think how many leaves are put out each spring by, for example, an oak tree, it seems an insane act of wastefulness to throw them away again at the end of the summer. Yet, such is the cost of maintaining a broad leaf in working order during unfavourable weather that it actually pays the tree to adopt the deciduous habit if its leaves are of that type.

In equatorial forests most broad-leafed trees keep a perpetual canopy of foliage, shedding leaves in small numbers all the year round. The same is done here in the temperate zone by a few broad-leafed trees, like the evergreen oaks, and the world over by pines, spruces, and the other “official” evergreens. In fact all trees everywhere regularly shed leaves which have reached the end of their working lives. Looked at like this, the autumn leaf-fall does not seem quite so profligate.

If a branch is killed, the leaves merely wither. They do not fall: the autumnal leaf-shedding is a carefully controlled operation, involving the least possible loss to the plant. Preparations for it have been going on since the summer.

Whenever a plant deliberately sheds an organ – be it a leaf, a petal, a whole flower, a fruit – a special separation layer is formed where the split is to take place. The layer is made of cells with an inherently weak jelly-like middle. Meanwhile, below this the tree forms a sealing layer which will keep out bacteria and fungal spores when the separation finally comes.

As the growing season reaches its end, the tree reclaims whatever it can from the leaf. Valuable trace elements are resorbed. The chloroplasts are gradually allowed to die, losing their green colour. The delicate conducting vessels which, all summer, have carried sugars out of the leaf, are one by one shut down and their point of entry into the system sealed off. All that remains are the tougher, woodier, water-conducting vessels.

Having lost its chlorophyll, the leaf is given colour by its residual pigments. Chief among these are xanthophylls and carotenes, which have an important job to play in the chemistry of the leaf. The autumn yellow of, say, the birch or sycamore is solely due to these.

There are other pigments, too. The golden yellow of beech leaves is produced by a brownish pigment, probably tannin, which is present in addition to the xanthophylls and carotenes. The more promiment reds and purplish-browns of the autumn woods come from a group of compounds called anthocyanins – which also give the red, purple or blue colour to many flowers.

The best autumn pageant comes only when the weather promotes the development of anthocyanins, when there are extended periods of bright, clear, dry weather with cool but not freezing night temperatures. This weather is the norm in autumn in New England, where the display of maples, oaks, and cherries attracts large numbers of admiring visitors every year.

The display in our own woods and gardens is not bad, either. Our native trees have a marvellous range of yellows, reds, and browns. The beech, of which different individuals turn at different times, offers perhaps the most beautiful show of colour, but such trees as the field maple, willow, rowan, and wild cherry can sometimes put in a really glorious performance. Then there are all the introduced trees, many of which have some fine autumn tints.

By the time the leaves are at this stage the whole metabolism of the tree has undergone a profound change. The hormonal balance – for plants, like animals, control their tissues with hormones – has tipped in favour of the dormins, a group of compounds prominent, as their name suggests, in the dormancy of seeds and buds. The growth-promoters of late winter and spring have been largely replaced by growth inhibitors. There is a build-up of these compounds in the area of the separation layer.

A leaf on the point of falling is held in place only by its separation layer, its outer skin, and the woody water-conducting vessels in the middle of the leaf-stalk. Wind and gravity between them work to free the leaf’s hold; the end often comes after a hard frost, which freezes the jelly-like middle of the separation layer, making it expand and forcing the cells apart.

Each breeze then thins the foliage, revealing more and more of the winter skein of twigs. The tree, the sugar factory, has shut down until spring: but next year’s leaves are already formed, crammed into buds.

When they reach the ground the old leaves are seized upon by an army of invertebrates and broken down into rich soil ready for future generations of leaves, and for future generations of trees and for all the other plants and animals that depend on them. So in the end nothing, really, is wasted after all.

(Introduction to these pieces; see all)