Water and Wood part 4

Trees which do not shed layers of dead tissues regularly have increasingly rugged bark as they age. This can often be observed by noting the relatively smooth nature of the top of a trunk in comparison with the older parts at the bottom. With age, the pressure from within causes the surface to crack and the resulting deep fissures are typical of many trees. The way trees develop such features can be diagnostic and one of the most distinctive is the Sweet Chestnut (Caslanea saliva) which usually cracks in spirals.

The corky layer in the bark, being impervious, not only prevents water and nutrients from getting to the outer cells but also blocks the passage of air to the inner tissues. As these tissues must have a supply of oxygen, the bark contains a number of tiny ducts called lenticels, through which air can pass. These replace the stomata which perform the same function in the living epidermis. The lenticels are filled with parenchymatous cells with numerous intercellular spaces which allow air to pass through. In winter they become closed by the growth of a layer of closely packed tissue which breaks away in the following spring. In trees with a persistent cork cambium layer the lenticels, once they are formed, remain active during the life of the tree. As the tissue expands with the tree’s growth so the size of the area covered by the lenticels increases and they become more conspicuous. The black lenticels of the Silver Birch are very obvious on older trunks and they are also responsible for the distinctive flecking of the bark of Cherry (Prunus avium). Trees which develop a new cork cambium layer every few years form new small lenticels within it, so they never become a feature of the bark patterning.

When a tree suffers damage, the cells around the area are capable of dividing and forming a protective layer. If the wound does not penetrate deeper than the layer of parenchyma, a new cork cambium will be formed beneath the damaged cells and will grow across the cut surface. If the damage goes deeper and vascular tissues are affected then the cambium cells within will divide and eventually form woody or corky tissue. A frequent injury to a tree is the breaking away of a branch leaving all the tissues exposed to attack by fungi and bacteria. Again the cambium cells, both the phellogen and those in the vascular tissues, will begin dividing and eventually form a protective layer across the wound, or, if the injury is very large, across the outer living wood. These scars are a familiar sight in forest trees.

When a tree is felled, its history is visible in the annual rings which tell of its periods of success or of difficulty, while the scars in the bark will indicate what damage has been inflicted upon it. In the wild, as an old tree in the forest falls, so other seedlings will begin to compete for the space, growing rapidly towards the light thus starting the sequence all over again.

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