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Leaves must produce food out of thin air, or else there is no tree. Luckily for nature and all of us, they are extraordinarily good at it. There is, for example, a stupendously beautiful oak tree at Ariundle, within the Sunart Oakwoods of coastal Argyll, that is perhaps eighty feet tall and of a still mightier girth of limbs. It is also an old acquaintance of mine. Consider first that the whole edifice is the work of its leaves, and that no leaf lives longer than six months. Then marvel at nature. Then believe in magic.
Leaves begin life tight-packed in a bud. In spring, they start to expand, then they start to draw the sap up through the tree.
How do they do that?
That is absolutely my favourite tree question. Because the answer is that no one knows. We can split the atom and fly to the moon and find water on Mars but we don’t know how a leaf drags a tree up into the air. I find that profoundly reassuring.
The containing scales of the bud respond to the pressure from within and hinge backwards allowing the leaves to open, at which point they go to work, which is food-shopping. Look again at the eighty-feet-high oak tree and take a wild guess at how many tons of timber it holds aloft in a crazy fan shape of idolatrous sun-worship. Almost all of it, perhaps as much as ninety-five per cent – the fabulous girth of the trunk and almost every bough, limb, branch, twig and twiglet – is nothing more than carbohydrates ensnared from the air by leaves. Before any one leaf is even half-grown, it has stored up more sustenance than it will need for the rest of its life, but it goes on food-shopping because that is what leaves are born to do, and it donates everything else throughout its life to the tree.
Photosynthesis insists that the action of sunlight on the leaf impregnates its water-filled vessels with chlorophyll (which, incidentally, is why leaves are green), a process that in turn exchanges hydrogen from the water with carbon and oxygen from the air. Photosynthesis needs a certain amount of evaporation to take place, but leaves are so super-efficient at evaporation that they deliver infinitely more than photosynthesis needs. As they lose water to the air they also draw it up through the tree, up through that trunk, through those boughs, limbs, branches, twigs and twiglets; they circulate sap all through the tree, they even draw water through the roots and out of the soil (dissolved soil minerals are the tree’s other source of food). Trees consume unimaginable quantities of carbon dioxide. This is why planting unimaginable quantities of trees will save the planet. Carbon is the tree’s primary food source, as well as the source of soluble sugars and starches that can be stored or converted to cellulose strengthened with lignin, which makes the thing we call wood, adding a ring to the girth of the trunk every year. This is why we can tell the age of a felled tree.
Then autumn kicks in, and all that stops. It stops because the leaf stops producing chlorophyll, with the immediate result that the green starts to fade. If you make a study of turning leaves on an autumn tree, you will see that the green survives longest in the veins. Green is replaced by a yellow pigment called carotenoid, or a red one called anthocyanin, or both, (and often it is noticeable how the sunniest parts of the tree change colour first and those in deeper shade linger greener for longer). When the yellows and the reds and the indeterminate pinks and oranges have had their fling, decomposed carbohydrates and oxidised tannins turn autumn leaves brown. In a tranquil and all-but-windless early autumn like 2015, huge clusters of leaves turned brown on the tree, and they would come away in your hand in their dozens and flutter uselessly at your feet, their show over, their race run.
ABOUT THE AUTHOR
Jim Crumley is a nature writer, journalist and poet with 30 books to his name. His 2014 book, The Eagle’s Way, was shortlisted for a Saltire Society award. The Nature of Autumn is published by Saraband on 25 August, price £12.99. He is at the Edinburgh International Book Festival on 22 August.