To most people, tree bark is not especially interesting or attractive. Scientific studies of bark are not very common compared to those about wood, a raw material essential in the world economy. But if you do look carefully at common tree barks - pine, eucalyptus, birch, plane, or palm - you will be amazed to discover rainbow colours and a great variety of textures: smooth, rough, cracked, prickly, spongy, corky.
Botanists use leaves, buds, and flowers to identify species. Rarely is bark considered an important key for classification. But bark is essential for distinguishing the 700 species and varieties of Eucalpytus. To identify these, Australians determined the genus according to different categories of bark: iron bark, box bark, stringy bark, smooth bark, tessellated bark, scribbly bark. That’s why Eucalyptus is my favourite tree.
This huge diversity of bark has evolved by natural selection over millions of years. Because trees are permanently located where their seeds germinate, they developed strategies to survive and protect themselves against animals, parasites, bacteria, fire, snow and environmental changes. The prickly bark of many acacias (Acacia karoo) in the African bush discourages big grass-eating animals from grazing on them. The bottle tree (Brachychiton rupestris), and the paper bark cork wood (Commiphora marlothii) both have green photosynthetic bark taking the place of leaves, which drop off during the dry season. Baobab bark (Adansonia digitata) is very spongy and therefore able to store water in its inner tissues. Giant redwoods (Sequoiadendron gigantQuercus subereum), paper bark trees (Melaleuca quinquenervia), and cork oaks () have very thick barks that are resistant to fire.
Bark is the skin of trees, which evolves day after day, season after season. The following animation illustrates the ageing of the Monkey puzzle tree (Araucaria araucana), from young specimen with prickly leaves, to thick trunk of a venerable old tree.
Bark is the skin of trees, which evolves day after day, season after season. The following animation illustrates the ageing of the Monkey puzzle tree (Araucaria araucana), from young specimen with prickly leaves, to thick trunk of a venerable old tree.
Due to growth of the trunk, environmental factors, and characteristics of each species, tree bark may split, peel, crack or rip. The outer layer of bark on one species may vary depending on the season, geographical situation, direction of sun rays, climate and soil conditions, and of course the age of the plant.
This makes it rather difficult to propose an exhaustive typology of tree barks. However, it is possible to compare the main characteristics of outer bark, and designate 14 categories :
Smooth and uniform bark (Fagus sylvatica, Celtis australis, Adansonia digitata)
Bark is a vital part of woody plants. It provides protection, transports nutritious substances, and eliminates wastes.
Protection: The vascular cambium is the most important part of the tree: This layer is composed of meristematic cells, which produce secondary tissues. Toward the outside they form the bast (secondary phloem) or inner bark; toward the inside, what we commonly call the wood (secondary xylem).
At the periphery, a short-lived meristematic layer called cork cambium (phellogen) produces other tissues: phelloderm toward the inside; and cork (phellem) toward the outside, containing a waxlike substance (suberin) that renders cells waterproof. The accumulation of these dead layers forms the outer bark, or rhytidome.
The main function of bark is to protect the vulnerable cambium. It can be a physical barrier against animals, snow, frost, UV rays or fire (cork oak & redwood have thick, inflammable bark) and a mechanical or chemical barrier against insects, parasites & bacteria using toxic substances such as latex or resins produced by the bark.
Nutrition:
The water pumped from roots with high minerals content represents the sap transported through the woody vessels to the leaves, supplying them with nutrients from the soil.
Then, with the help of sunlight, assimilates are produced through photosynthesis. These are conducted from the leaves to all parts of the tree via the inner bark: the bast. Characteristic green barks of certain trees growing in dry conditions can store chlorophyll in their phelloderm, allowing them to continue photosynthesis even without leaves.
Purification:
Trees eliminate noxious substances (excess metabolites) in the form of resins, tannins, etc. These accumulate in the bark, which dies and progressively comes off the trunk.
To summarize: Bark is composed of living cells and tissues (meristematic layers, phloem and phelloderm), but also dead structures (rhytidome), which can remain fixed for a long time on the trunk or peel off yearly.
Thanks to their huge diversity, tree barks are used in many fields :
Medicinal properties:
For a long time, illnesses have been cured using bark :
- Aspirin (acetylsalicylic acid), derived from Salicin, is a substance found in the Salicaceae family (Salix-willow, and Populus-poplar genus). Pieces of dry willow bark can be boiled & the infusion used to relieve headaches and rhumatism.
- Quinine bark (Chinchona pubescens) for many years provided the only effective treatment against malaria. It contains a large quantity of 2 powerful alkaloids: quinine and quinidine.
- The Vezo, a Malagasy tribe, crush the bark of a local Ficus called Amontana (Ficus baroni) on a stone, then add water to obtain a natural sun cream that protects their skin from sun burn.
- Baobab bark is sold as medicine to prevent calcium deficiency at the local markets of west Madagascar.
Building:
Some barks have excellent insulating properties. The phellem of the cork oak (Quercus suber) is the best example. The virgin cork called “male” is used for the fabrication of particle boards or insulating panels. After regeneration, the new cork called ”female” is a better quality and used for making bottle corks. The flaky bark of the paper bark tree (Melaleuca sp.) is used by the Aborigines of Australia to build their shelters. The inhabitants of the French island New Caledonia use the same bark for their traditional huts. The Kauri resin (Agathis australis) was for a long time the best boat varnish.
Energy source:
Bark is an excellent fuel. When dry, its density (about 350kg/m3) is twice as light as wood, for nearly the same calorific power (about 18 000 kJ/kg).
Horticultural uses:
Scaly barks, especially Pinus pinea bark (Umbrella Pine), are used in many different horticultural preparations: substrates for orchids, mulches to retain moisture and prevent the germination of weeds, additives to improve the structure and quality of soils, etc.
Textile and craft:
Certain barks are very rich in fibers. After treatment, barks supply fibers used for making baskets, ropes, mats and even clothes.
Other barks are used to make paper, as indicated by their Latin names: Betula papyrifera and Broussonetia papyrifera. Antemoro paper is a traditional Malagasy paper still produced in Ambalavao. It is made from crushed Havoha barks (family of mulberry trees). Softened in water, the bark slurry is spread onto linen trays. Each sheet is then decorated by hand with fresh wild flowers and other parts of plants. The paper is then left to dry for 1 day in the sun.
Comfort and pleasure:
Certain trees have aromatic barks. The best-known is Cinnamomum verum or cinnamon used in delicious recipes. Others are used for their fragrance, to make incense sticks: Santalum album or Sandalwood. Resins produced by the bark of Commiphora or Boswelia are collected to produce varnish, shellac, lacquers, and are also the source of incense such as myrrh and copal. Certain fossilized resins called “amber” are considered precious and used for making jewellery.
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