How trees age and what we can learn from them

[Book extract] A break in its bark, then, is at least as uncomfortable for a tree as a wound in our skin is for us.

 |  5-minute read |   26-09-2016
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Before I talk about age, I would like to take a detour into the subject of skin. Trees and skin? First let's approach the subject from the human point of view.

Our skin is a barrier that protects our innermost parts from the outer world. It holds in fluids. It stops our insides from falling out. And all the while it releases and absorbs gas and moisture.

In addition, it blocks pathogens that would just love to spread through our circulatory system. Aside from that, it is sensitive to contact, which is either pleasant and gives rise to the desire for more, or painful and elicits a defensive response.

Annoyingly, this complicated structure doesn't stay the same forever but gradually sags as we age. Folds and wrinkles appear so that our contemporaries can playfully guess how old we are, give or take a few years.

The necessary process of regeneration is not exactly pleasant, either, when looked at close up. Each of us sheds about 0.05 ounces of skin cells a day, which adds up to about a pound a year.

The numbers are impressive: ten billion particles flake off us every day. That doesn't sound very attractive, but sloughing off dead skin is necessary to keep our outer organ in good condition.

And in childhood we need to shed skin so that we can grow. Without the ability to renew and expand the covering nature gives us, sooner or later, we would burst. And how does this relate to trees? It's just the same with them.

The biggest difference is simply the vocabulary we use. The skin of beeches, oaks, spruce & co is called bark. It fulfils exactly the same function and protects trees' sensitive inner organs from an aggressive outer world.

Without bark, a tree would dry out. And right after the loss of fluid, fungi - which have no chance of survival in healthy, moist wood - would go to town and start breaking everything down.

Insects also need lower moisture levels, and if the bark is intact, they are doomed. A tree contains almost as much liquid inside it as we do, and so it's unappealing to pests because they would, quite simply, suffocate. A break in its bark, then, is at least as uncomfortable for a tree as a wound in our skin is for us.

And, therefore, the tree relies on mechanisms similar to the ones we use to stop this from happening. Every year, a tree in its prime adds between 0.5 and 1 inch to its girth. Surely this would make the bark split? It should.

thehiddenlifeoftrees_092616092846.jpg The Hidden Life of Trees  (Indian edition introduced by Pradip Krishen); Penguin; Rs 499.  

To make sure that doesn't happen, the giants constantly renew their skin while shedding enormous quantities of skin cells. In keeping with trees' size in comparison to ours, these flakes are correspondingly larger and measure up to eight inches across.

If you take a look around on the ground under trunks in windy, rainy weather, you will see the remains lying there. The red bark of pines is particularly easy to spot. But not every tree sheds in the same way. There are species that shed constantly (fastidious people would recommend an anti-dandruff shampoo for such cases). Then there are others that flake with restraint. You can see who's doing what when you look at the exterior of a tree.

What you see is the outer layer of bark, which is dead and forms an impervious exterior shell. This outer layer of bark also happens to be a good way of telling different species apart. This works for older trees, anyway, for the distinguishing characteristics have to do with the shapes of the cracks or, you could say, with the folds and wrinkles in a tree's skin. In young trees of all species, the outer bark is as smooth as a baby's bottom.

As trees age, wrinkles gradually appear (beginning from below), and they steadily deepen as the years progress. Just how quickly this process plays out depends on the species. Pines, oaks, birches, and Douglas firs start early, whereas beeches and silver firs stay smooth for a long time. It all depends on the speed of shedding.

For beeches, whose silver-grey bark remains smooth until they are 200 years old, the rate of renewal is very high. Because of this, their skin remains thin and fits their age - that is to say, their girth - exactly and, therefore, doesn't need to crack in order to expand.

It's the same for silver firs. Pines and the like, however, drag their feet when it comes to external makeovers. For some reason, they don't like to be parted from their baggage, perhaps because of the additional security a thick skin provides. Whatever the reason, they shed so slowly that they build up a really thick outer bark and their exterior layers can be decades old.

This means the outer layers originated at a time when the trees were still young and slim, and as the trees age and increase in girth, the outer layers crack way down into the youngest layer of bark that - like the bark of the beeches - fits the girth of the tree as it is now. So, the deeper the cracks, the more reluctant the tree is to shed its bark, and this behaviour increases markedly with age.

(Reprinted with publisher's permission.)

Also read - We need to pass our love for nature to children

Writer

Peter Wohlleben Peter Wohlleben @peterwohlleben

The writer is a forester.

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