The Living Skin of a Very Old Corpse

Lean against an oak sometime – really lean on it, put your full weight against the bark – and you are resting against something that is mostly dead. Not dying. Not struggling. Just dead, and has been for centuries.

A Tree Grows Outwards, Not Upwards

This tends to surprise people, because we think of a tree growing the way an onion forms: new layers adding inward, building up around some tiny original core. But it works the opposite way. A tree grows outward, adding new wood to the outside of what was already there. And the moment that new wood hardens and takes on its structural job, the wood beneath it quietly stops being alive. Last year's growth becomes scaffolding. The tree moves on, wrapping another living layer around the outside while the stuff beneath becomes part of the permanent skeleton – which is what we call heartwood, and it has been dead for decades, sometimes centuries, by the time the tree is anything to look at.

So the oak in the park, the one that was already old when your great-great-grandparents were young: the broad dark column of it, the part that looks so solid and enduring, is a cathedral of dead wood. The living bit is a skin. Just a few cells thick, sitting between that dead scaffold and the bark. Roughly the thickness of a decent piece of card. That is your tree. The rest is just the frame.

Why do hollow trees carry on standing?

This matters because it completely changes how you read a hollow tree. People assume a hollow trunk means a dying tree, a candidate for the chainsaw. In fact, a tree can lose its heartwood entirely – to fungus, to rot, to a slow centuries-long process of digestion from the inside out – and carry on in perfectly good health, because the hollow has nothing to do with the living part. The thin green cambium layer beneath the bark is still intact, still doing everything a tree needs to do: moving water and sugars around, laying down new growth, photosynthesising quietly up in the canopy. Some of Britain's oldest trees – ancient pollarded oaks, veteran limes in churchyards – are entirely hollow and entirely thriving. (For more on how living things are frequently not what they appear on the surface, *The Pound in Your Pocket Is Not the Same Pound That Was There Last Year* is a good wander through a similar idea applied to money.)

Does dead heartwood still do anything useful?

There is something almost pleasing about this once you sit with it. The tree is not preserved. It is not holding on to what it was. Every year it simply moves the living part outward, leaving its past behind as load-bearing structure, and keeps going. The heartwood does its job better dead than it could ever have done alive – harder, denser, immune to the squashing that living cells would buckle under.

What looks like endurance is mostly just excellent scaffolding.