Tree Bark: A Protective Skin
Bark – whether it be rough and fissured as on an oak, or smooth as on a beech – performs the same function in all trees: that of protecting the delicate living tissue in the trunk from the weather and from attack by animals and diseases.
Even the most casual visitor to a public garden will have noticed the immense variation in the barks of trees. The smooth, brilliant white bark of the Himalayan birch and the glossy, reddish-brown, peeling bark of the Tibetan cherry make these two of our most attractive ornamental trees. Among our more familiar species, the sweet chestnut has a strikingly spiralled bark, while that of the Scots pine is a rich brown and falls away from the tree in flakes. To us, the bark of a tree is often a very useful aid to identification, particularly in winter when, many trees have lost their leaves. For the tree, however, the bark performs a vital function in protecting it from extremes of weather and against the attacks of a variety of agents, from bacteria and fungi to rabbits and deer.
How bark functions
Bark consists of a living inner layer of tissue (the phloem) and an outer dead layer. The phloem plays a vital role in transporting sugars down from the leaves, where they are manufactured by photosynthesis, to the roots and other parts. The outer layer of bark is waterproof and so protects the underlying tissues from drying out. In parts of the world where seasonal fires are a potential hazard some trees have extra-thick barks to insulate the living tissue from the heat. The best known of these is the cork oak from the Mediterranean region, the bark of which has thermal insulating properties rivalling those of glass fibre. Furthermore, the presence of air spaces makes the material light and compressible. Combine this with its waterproof qualities and you have the ideal material for sealing bottles – cork. The cork oak is not often seen in British parks, but its hybrid with the Turkey oak, called Lucombe’s oak. Appears much more frequently, especially in the West Country.
Bark is also a tree’s first line of defence against attacks by bacteria, fungi, insects and larger animals such as deer and rabbits. In most cases it repels these attacks effectively. Though large herbivores may destroy so much of a tree’s bark in winter when other food sources are scarce that the trunk is completely encircled by the damage. When this happens the tree inevitably dies.
To provide extra protection against this possibility, trees with thin barks higher up often have much thicker barks near the base. This may be seen on both silver birch and Scots pine.
Bark is formed by the activity of a special sort of cambium (the wet, green tissue lying just underneath the bark). Known as the bark cambium, this layer plus the corky cells it creates are together known as the periderm. In the young tree, the periderm first arises in the outer tissues of a shoot and can be seen as a colour change, usually from green to grey. The colour change is caused by the presence of waterproof waxes and other materials in the walls of the bark cells. In some species, such as beech and hornbeam, the periderm thus created can last for many years, slowly increasing in size by cell division as the tree’s girth expands. In the majority of trees, however, the first periderm is soon followed by others arising from progressively deeper layers of the stem. With the formation of each periderm, the tissue layers lying outside it are cut off from their source of water and nutrients, and so die.
Different bark types
As the branches and trunks increase in girth resulting from cell division in the cambium, the bark must also increase in size. Thus new layers of bark are continually created within the stem. The number of layers of cork cells produced by the periderm and the depths at which they occur in the stem vary considerably and determine the thickness of the bark. A thin bark, such as that of Stewartia species, is smooth while a thick bark, such as that of English oak or black walnut, is rough and fissured. In some trees the older layers of bark readily peel away or break off. This can be seen in the paper-bark maple and various species of strawberry tree. If the layers remain firmly attached, as in wellingtonia and coast redwood, then a thick bark builds up, which is only gradually worn away. Persistent barks often become deeply fissured because the older, dead, layers on the outside are unable to grow to keep pace with the expanding trunk.
In some species the periderm can completely encircle the stem and, if the outer bark falls away, then whole cylinders of bark can be discarded. Traveller’s joy, honeysuckle and paper birch all have such barks. In the case of paper birch, large sheets of its bark were used by certain North American Indians in the building of their canoes. In other trees the periderm is much more localised, arising one beneath another like overlapping scales. The resulting bark is therefore shed as discrete, irregularly shaped pieces. In Britain, the most familiar examples of this type of bark are those of yew and London plane.
Lenticels for air
In common with herbaceous plants, the stem of a young tree contains structures known as stomata through which the stems cells obtain carbon dioxide and lose oxygen. When the stem loses its original epidermis it also loses its stomato and their job is taken over by small lens-shaped pores in the bark called lenticels.
Most woody plants have lenticels on their stems, though they can vary from being microscopically small to 1cm (1/2in) or more in diameter. In some species, such as birches, the lenticels enlarge with age.
Along with lenticels, some trees also have spines on their branches or trunks. Hawthorns have spines formed from small branches, though at up to 1.5cm (5/8in) long they are insignificant compared with the branched thorns of some honey locusts, which can grow to a length of 10cm (4 in). The more familiar locust tree, or false acacia, has spiny stipules on its younger branches. These were originally at the bases of the leaves.
Occasionally trees produce masses of shoots directly from the bark. These arise from buds that, for most of the time, lie dormant within the trunk. This is a particular feature of the common lime, which often has large tufts of leafy shoots sprouting from its bark. Dormant buds can be very important to the tree as a means of regeneration after a forest fire, damage by lightning or even heavy. The Judas tree is unusual in that it frequently produces bunches of directly from its bark.
While some trees are grown for their ornamental barks, other trees have barks that are of considerable commercial importance to us. One of the most familiar bark products is latex, which is secreted by specialised cells arising in the phloem. By making incisions in the bark of certain trees the latex can be extracted and converted into products such as rubber and the gum of chewing gum. Few trees growing in Britain yield latex, but one such is the fig.
Some trees are tapped for the sugary sap carried in their phloem. The sugar maple, for example, yields maple syrup while in southern Europe several species of pine are tapped for their resin. Certain wines are flavoured with resin from the Aleppo pine. Other resin products from pines include rosin and turpentine.
Another very important source of bark products are materials produced by the tree inside the bark cells themselves, mostly as a form of chemical defence against animals and other potential attackers. An example is tannin, high levels of which are found in the barks of oak trees, making them important in the tanning of hides for leather.
Finally, a few barks are used as a source of aromatic products. Perhaps the best known of these is the spice cinnamon, which is the inner bark of a tree from south-east Asia. The scent myrrh is also extracted from the bark of a tree, in this case by tapping the bark and extracting the myrrh from the resin.