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As I drove home in the aftermath of Storm Ali ten days ago, avoiding branches and the occasional traffic cone, I could see the council had been busy making safe the fallen trees at the sides of the roads. With the trees still in full leaf and acting like huge sails, I knew this unseasonably early storm would have big consequences for some poor trees and my thoughts turned to our beautiful Common Lime tree. Affectionately known to us as ‘The Big Guy’, he dominates our tiny garden and features on the front cover of my book The Plant Listener.
The omnipresent Big Guy dominates the view from our kitchen window. In late spring, he clothes himself with fresh, bright green leaves and signals the start of those precious, warm summer months. Every year, he is host to a family of blue tits and, all year round he is a safe perch for all the birds who visit our garden. In the evening, the brown rats who live under next door’s shed, hide behind his trunk before attempting raids on the bird table and, all through the twilight hours of summer, the pipistrelle bats flit above his canopy catching the midges that swarm around him. He shades out most of our garden and dumps so many leaves in autumn he has completely ruined our tiny lawn, but we wouldn’t be without him.
As soon as I got home on the night of storm Ali, my first question was ‘How did The Big Guy get on?’ My partner knew exactly to whom I was referring – The Big Guy had taken a beating but he was okay and luckier than the tree across the road which had been completely ripped in half.
Despite having survived the storm, Ali has got The Big Guy worried. He’s going for a speedy leaf-dump just in case he’s caught out like that again. He is, as we horticulturalists would put it, senescing.
Deciduous trees, like The Big Guy, go through this autumn senescence every year. It is a natural, programmed response to the coming of winter in order to protect themselves from wind and snow. It also serves to reduce the amount of food available to potential predators at a time when the tree is at its most vulnerable. Without his leaves, The Big Guy should be able to take storms much worse than Ali in his stride.
Trees, and other higher plants, rely on a conducting system which cycles water and nutrients around the plant. A plant’s vascular system extends all the way into the leaf veins so, before the leaves are dumped, The Big Guy must ensure he applies a tourniquet to prevent the loss of water and sugars, and to ensure there is no open wound which might allow diseases to enter. He does this by instructing a layer of cells at the base of each leaf stalk to thicken – the abscission layer. With the supply of water and sugars cut off from the leaves, they begin to lose the chlorophyll that gives them their green colour.
When the chlorophyll is lost, other pigments, which have essentially been hiding behind the green, begin to show. This is when we get that wonderful display of autumn colours. Ordinarily, The Big Guy’s leaves turn a bright yellow before they fall due to carotenoids and xanthophylls in his leaves. Behind him, the Acers lining the street turn a gorgeous shade of red due to a high amount of anthocyanins in theirs. But Storm Ali has already sucked most of the moisture out of his leaves so he’s gone straight to rusty brown and a faster leaf drop.
It won’t be long before The Big Guy drops all his leaves and takes a well-earned winter rest. On the ground at his feet, the leaves will slowly decompose, providing a rich food source for all manner of creatures and other plants. During the long winter months, I will hang fat-balls from his branches to help the birds see through the winter and watch for the return of the blue tits, the re-awakening of the bats and the breaking of his buds into promising bright green leaves again.
Autumn colour image by By DigbyDalton [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)], from Wikimedia Commons