Autumn foliage is a kind of alchemy

A DECIDUOUS tree is really a tree that has learned to hibernate, going through profound metabolic changes in order to survive the winter.

The reason there are no deciduous trees in the tropics is because it never gets cold there.

It can get cold in Ireland, certainly, but because of our relatively mild maritime climate this normally only happens quite late in the year. For this reason, our deciduous trees “hibernate” later than most. I once made a TV series about trees for an English production company. The director wanted to fly over to Ireland to film autumn colour. He couldn’t believe it when I told him to delay his flight until well into November.

To understand the metabolic changes responsible for the autumn colour surrounding us at present, you have to appreciate some of the odder facts about trees.

First of all, the average tree — certainly the average deciduous tree — has as much mass below the ground as above. Some of this is in fine root hairs which would not survive the pull of gravity above ground, but there is as much weight of wood below ground as above — which, when you contemplate a massive oak, beech or ash, is a fairly astonishing fact.

How do I know this? Well, some English researchers once painstakingly exhumed a large Bramley apple tree and then weighed the root matter, the trunk and the branches.

What’s going on at present is that deciduous trees are sucking every bit of life out of their above-ground mass and storing it in the below-ground wood until the weather gets warm again in the spring. For the next few months, everything you can see in a deciduous tree (unless you dig it up) is literally dead wood.

The leaves of trees are really solar panels. By the miracle of photosynthesis, they turn sunlight and carbon dioxide into tree food and oxygen. The catalyst for this alchemy of molecule splitting is, of course, chlorophyll, and chlorophyll is green. This is why most leaves, most of the time, are green. Copper beeches or variegated poplars have been bred by humans, and their chlorophyll has been tampered with, which compromises their efficiency. A green beech will outperform a copper beech every time.

The chlorophyll, dissolved in a sugar solution, is travelling downwards in the cambium, which is that slimy layer between the bark and the wood, for safe winter storage underground. When it’s sucked out of the leaves, residual cellulose colours remain; they were there all the time, but were drowned out by the chlorophyll green. Brown is the commonest, but there’s also yellow, red, orange, even purple. This is autumn colour.

All of this is reasonably straightforward. But one of the reasons I am fascinated by trees, why I’m spending so much time this autumn admiring and wondering, is that aspects of this annual miracle are rather less straightforward.

It’s convenient to compare the circulatory system of a tree to that of a mammal. The sap is its blood and the fine tubes in its cambium are its arteries, veins and capillaries. But the tree does not have a heart. There is no pump to circulate the sap, no mechanism to suck hundreds of litres of fluid underground.

Even more miraculously, there is no pump to get this life back up into the sky next spring. There are scientists who claim to have an explanation for this mystery. I don’t understand their explanation and I don’t totally believe it. This leaves me wondering at the magic of it all — which I quite enjoy.

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