Bark

Have you ever been to a quiz where you’re asked what the largest human organ is? Depending on which authority / point of view you take, one expected answer is ‘the skin’. The only reason I mention it is that it is often overlooked as such a large part of our bodies – apart from skin care product manufacturers.

I think maybe the same is true for bark on trees. There are some similarities to our skin – some of it, the outer layer, is dead – and there are some major differences. Our skin does not provide an important role in circulation the same way as bark does for trees. Some of the living part of bark, the phloem, transports sugars away from leaves to other parts of the tree. That way the roots, for example, can have energy to grow, metabolise and do all the work that roots do whilst being in the dark, without chlorophyll and unable to produce their own sugars via photosynthesis. So part of the bark is like tubular conducting cells, through which dissolved substances can flow and this system grows and expands with the tree. The layer responsible for this growth is called the Cambium which produces undifferentiated cells – like stem cells, I suppose. These cells then decide whether to become a conducting cell or a supporting structure cell or whatever else makes up the phloem. Phloem is the proper botanical term, bark more of an everyday term and is clearly what we recognise as the outside of a tree, maybe without giving a second thought to what’s happening inside.

Except, I remember being told as a young boy, that if you cut all the way around the bark on a tree it will kill the tree. I think you would have to cut off a thick enough strip such that the tree could not heal across the gap and this would then cut off the flow of sugars and you can see why it is not a good thing to do.

As for recognising the outside of a tree, guide books often use the appearance of bark as one of the keys to identifying trees and some barks are very distinctive. Not that I’m all that good at differentiating between some types of bark but I can generally tell a Silver birch when I see one – ok that’s an easy one; so are cherries, maybe

Suppose you have a bark that’s fissured like this one, below, which happens to be an oak.

Or it might have looked like the next one . . .

. . . which was a cedar. They are both very different and I think I would pick the first one as being an oak but not sure I’d know the second one was a cedar. And the next one . .

. . . was a Goat willow and it is the rear trunk that looks more fissured than the front which has characteristic diamond markings but I can’t seem to find this in my key system. maybe I should work backwards to find out how the key works!

Lets try an easy one, the London plane

Often called the camouflage tree because of its distinctive bark, the London plane is quite easy to recognise. So are birch trees and the first picture shows a young Silver birch

Followed by an older Silver birch . .

. . . which starts to show more scars and splits in its bark. Then there is the Himalayan birch . .

. . which is altogether more delicate and paper-like in appearance and the lenticels (horizontal stripy things) are smoother. Speaking of lenticels, they are what make cherry bark distinctive. By cherry I mean Prunus species in general which may cover more than just cherries.

Here the lenticels have aged into horizontal banding and the smooth bark normally seen has started to give way to more fissured bark. Not a typical cherry bark.

There are quite a few trees that have bark which peels, some peels like paper such as the Himalyan birch or this one . . .

. . . the paperbark maple. It has many shades of ruddy brown changing as the layers peel off in delicate thin strips. Whereas, this Eucalyptus loses its bark in great big chunky curls. At the moment there are only a few pieces left clinging to the trunk as the recent high winds have picked it near clean and the ground is littered with coils of discarded bark.

Not quite so paper-like but probably described as scaly is the bark of the Yew tree. As well as being scaly and flaky Yew bark is often found inside the tree. I think it’s a characteristic of the way the tree grows in on itself and multi-stemmed trees often merge their stems and what was once on the outside ends up on the inside. As a woodturner, who loves turning yew, I’ve often found bark inclusions in pieces of yew or found that one piece of yew actually becomes two again when you reach the bark inside.

My last two pictures are of a Ginkgo . .

and a Black mulberry . .

These last two were simply to contrast colour. The Black mulberry has such a range of vibrant colours, only some of which are shown in the picture above. The mulberry is alive with colour whereas the bark of the Ginkgo has the appearance of a plate of cold porridge. It’s such a lovely tree but its appeal is not in its bark.

Anyway, I am sure I will still find bark alone a difficult means of identifying trees but it is, nevertheless, a fascinating subject. When better weather arrives I must give some serious thought to organising some bark rubbing activities; I’m sure that Black mulberry would be a challenge. There’s also a lot more to be said about bark and much more I could discover; I certainly have only included a few of the trees from Kings Heath Park. In fact, if you’ll pardon the pun, I’ve barely scratched the surface.

Talking of scratching the surface, one last picture; a cross section of some bark from a Silver birch that came down in storm Dudley. There is a layer around 2.5mm thick just below the surface then a more spongy layer of around 10mm and a very thin black layer just before the bark ends and the woody xylem begins.

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