Dead armour over a living pipe
What Is Bark For?
The rough outside is dead. The thin layer just under it is alive, and cutting a ring through it kills the tree.
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Step 1 of 5
The rough outside is not alive.
Outer bark is dead cells packed with waterproofing. It cracks and flakes because it cannot grow — the tree keeps making more from underneath.
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Step 2 of 5
It is doing several defensive jobs at once.
It holds water in, keeps insects and fungi out, and insulates the living wood. Some barks are thick enough to survive a passing fire.
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Step 3 of 5
Just inside it is a thin living layer.
That layer carries the sugar made in the leaves down to the roots. It is only a few millimetres thick and it is the tree's only route for food.
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Step 4 of 5
Which is why a ring cut round a tree kills it.
Cut a strip of bark all the way round and the roots are left with no food coming down. The leaves stay green for months, and then the whole tree dies.
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Step 5 of 5
Every trunk is fatter than it was, from the same seam.
A thin layer between bark and wood adds wood on its inside and bark on its outside every year. That is how a trunk thickens without splitting open.
Cork is the outer bark of one oak, cut off carefully enough to leave the living layer beneath untouched.
The short version
Bark is dead waterproof armour over a thin living layer that carries food down — which is why cutting a full ring round a trunk kills the tree.
Try it yourself
Look at where a branch was cut off a tree. The roll of new growth around the wound comes from the living seam just under the bark.