The great divide

As I look across to the edge of High Wood from the back of the Biology department where I work I should be able to keep a watchful eye on my oak tree but it’s rather difficult to identify from the line of trees on the horizon.

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Having a few minutes to spare yesterday afternoon, and being in need of some fresh air, I decided to make sure I could identify my tree from a distance. From close up, it’s easy – the trunk divides into two a couple of metres above the ground. Depending on the angle you view it from, the two trunks seem to split apart but then come back together near the crown of the tree.

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Why is my tree growing like this? When an oak tree is but a seedling it exhibits what is known as apical dominance; that is, a single bud at the top of the seedling produces the plant hormone auxin which passes down the tree and inhibits the growth of side shoots by maintaining bud dormancy. This encourages the seedling to grow tall and straight – after all, in its natural habitat an oak seedling will be competing with larger trees around it for light so it is advantageous to grow upwards as quickly as possible. As the tree grows taller and the apical bud gets further from the side shoots below, the concentration of auxin reaching them falls. Dormancy is broken and the shoots grow out into new branches. This is what gives the traditional ‘Christmas tree’ shape to conifers.

If the apical bud is damaged at some stage early in the tree’s growth, maybe by insects or frost, the bud can no longer produce auxin and the lateral buds which were previously suppressed will start to grow out very rapidly. Some of them will grow upwards and they will fight it out for the dominant apical position. If two branches compete more or less equally you end up with two apical buds and the sort of split trunk my tree sports.

You can see this apical dominance in action at a smaller scale on the branches too – the large bud at the apex of this twig is keeping the smaller buds beneath it in check. They will only grow out if the apical bud is damaged in some way.

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Anyway, I walked away from my tree towards work and have now got its location worked out so that I can spot it even from beyond Mountjoy pond. The tall larch just to its right helps enormously.

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I’m eager now to see my tree with leaves on though I have a sneaking suspicion, on the basis of its shape, that it is going to turn out to be a sessile oak (Quercus petraea) rather than an English oak (Quercus robur). We shall see.

 

 

 

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