Tree bark

I don't suppose this is a subject that many of us have given much thought to. It's just there, like the trees it adorns. We have probably noticed that various trees have different looking bark; the whitish bark of the silver birch being one of the more obvious. However, having researched it, I've discovered that there's much more to bark than meets the eye, and familiarity with the different types is one way to help identify trees, especially in the winter.

So what is it?....beyond the obvious! Slice horizontally across the trunk of a tree and you can see that it consists of various concentric rings (see diagram).
 

A thin band of actively dividing cells sits not far below the outer surface, and is called the cambium. This is the area that produces most of the cells needed to sustain the tree. On its inner surface it produces xylem, which are vertical pipe-like structures carrying water and nutrients up from the roots to the stems and leaves. These are constantly being replaced, compressing the older ones in the process. As they cease to function, the old xylem and the tough lignin, which makes up a good part of their structure, is pressured into dense hardwood. This is the core of the trunk and constitutes the main support of the tree. New cells are produced throughout the growing season; these add to an increase in girth, and are each represented by a growth ring.

But it's the substance that exists around the outer edge of the cambium that interests us, and its various layers together, constitute what is generally known as the bark. The cells produced by the outer ring of the cambium are known as phloem. This interconnected web of vascular tissue transports sugar and nutrients which have been produced by photosynthesis in the leaves. It carries them to whichever part of the plant requires them, for growth, respiration or storage. 

_{Horizontal section through a dead tree stump showing from bottom to top: the heartwood,
the xylem (sapwood), the cambium, the phloem (inner bark) and the outer bark.}

This constitutes the inner bark. The outer bark is what we see and is made up of dead phloem and cork; both cork and more phloem are produced by a layer of cells just below the surface, and their ongoing production, forces the dead material outwards to form the 'skin' we think of as bark. Our tree trunk is rather like a stick of rock with its pink outer coating being the bark. Although in this case, more and more layers are being produced internally and we have an ever expanding stick of rock!

The purpose of bark is to protect the tree against extremes of weather, fungal infections and attacks from insects and other birds and animals. The bark also contributes to the tree's absorption and excretion of gases. Trees are invaluable in taking up CO2 from the atmosphere and 'breathing out' oxygen; a process essential to life on earth. Although outer bark is made up of dead material which is impervious to gases, it has pores, often lens shaped, which occur throughout. These are called lenticels. Sometimes they're small and not easily visible, but they are more obvious on trees such as Cherry and Silver Birch, as the photos below show.

_{Left: the bark of a Silver Birch, that will flake off in time. 
Right: the smooth bark of the Beech - Note the lenticels}

There are broadly speaking two different types of bark. One is called smooth or peeling and the other, ridged or plated. The former includes fairly fast growing trees such as Birches (peeling) and Beech (smooth). In Beech, for example the bark tends be thin and expand outwards with the growing trunk, whereas Birches, shed their bark in peeling strips. 

_{The plated bark of the Horse Chestnus, and the ridged bark of the Crab Apple}

This is thought to be beneficial in removing a build up of moss, lichens and anything else that might impair the ability of the bark to allow the passage of gases, and also light, which the trunk apparently does! The resultant bark in both cases, is smooth. 

By contrast, in trees where the old bark is retained, but not able to expand, it becomes cracked as its pushed outwards, giving the appearance of ridges or plates. Oaks have ridged bark and Horse chestnuts, plated. These trees tend to have a faster growth rate which enables them to repair themselves more quickly. Each species of tree is adapted to its preferred environment.

Bearing in mind that bark does age and change its appearance as the tree matures, it can still be one of a number of clues to its identification. So next time you pass a tree, think of all the complexity of form and function that's going on within its trunk......then step back and admire its beauty, or just give it a big old hug!

_{Words and pictures by Denise Long}