To better understand the world of barks…

What is bark? What is its origin? What service does it provide for the tree? Do all trees lose their bark? Why is some bark so colorful? Why are there so many different kinds of bark? How does man use bark?...You will find the answers to all these questions on the pages that follow. This will allow you to better understand the complexity of this structure and the fundamental importance of this actual tree skin.

The word bark is also used here to denote the outer epidermis of some giant herbaceous plants, (palms, ferns, bamboo and other monocot trees) even though this structure is very different from that of trees.

Formation and composition of bark

Bark is the skin of trees, which evolves day after day, season after season. Due to growth of the trunk, environmental factors, and characteristics of each species, tree bark may split, peel, crack or rip.
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.

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.

The main functions of bark

Bark is a vital part of woody plants :

- Protection : 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 or secondary phloem.

- 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. It is also the possibility for certain trees to clean themselves getting rid of epiphytic plants (lichens, mosses, orchids…).

Do all trees lose their bark?

The possibility of losing its bark or not is directly related to the structure itself. Everything, therefore, depends on the species.

The great majority of trees native to our forests develop a scaly rhytidome (dead, outer bark) said bark becoming more or less thick from the accumulation, year after year of scales of various shapes, (larch, oak, spruce, pine...). The pressure generated by the increase in girth of the tree will gradually produce fissures, grooves or crevices. With age, certain vulnerable plaques can be detached from the trunk.

In contrast, trees with an annular rhytidome will tend to lose their bark regularly in shreds, (birch, eucalyptus, arbutus...). Like skin shed from a snake, with growth, these trees produce a thin bark that will tend to detach from the trunk. This exfoliation, closely tied to the seasons, is more or less marked depending on the species. It can be extremely rapid: in a hot dry period, the bark of an adult Arbutus andrachne, (Grecian strawberry tree) can flake off entirely in just a few days

Why are barks colorful?

One can easily understand the role of colors in flowers, fruits and seeds closely involved in plant reproduction. But what interest do trees have in coloring their trunks and branches green, red, yellow or sometimes even blue? This remains a puzzle for many experts.

You can, however, find some explanation for the color of the bark of certain characteristic species. The cinnamon colored bark of the sequoia is due to high tannin content. Some xeric plants have green bark due to the accumulation of chlorophyll in their cellular tissue. Thus, during the dry season, the tree loses its leaves to reduce water loss and the bark takes over the work of photosynthesis.
The Brachychiton rupestris, (Queensland bottle tree) or Commiphora marlothii, (paperbark corkwood) are excellent examples.

Finally, be aware that within one species, the color of the trunk may change depending on many parameters: age, period of exfoliation, the orientation relative to the sun, (trees can get sun-burned!!) or even the soil and climatic conditions (high relative humidity will saturate the colors and increase contrast).

Why are there so many different kinds of bark?

The kingdom of trees is immense : there are over 100 000 species, each with their characteristic wood and bark. This morphological diversity can also be explained in part by the ability of trees to adapt and respond to changes in their environment.

Many acacia have thorny bark that discourages most herbivores from grazing on them. The giant sequoia, (Sequoiadendron giganteum), the niaouli (Melaleuca quinquenervia) and the cork oak (Quercus suber) have very thick bark protecting them from fire. The tabaquillo (Polylepis australis) or birch (Betula sp.) have many-layered bark acting as insulation against the cold. Still others must defend themselves against attacks from parasites and other insects by releasing harmful substances (latex, resin) at the slightest injury.

This makes it rather difficult to propose an exhaustive typology of tree barks. However, it is possible designate 15 categories with : horizontal sections, shallow fissures, many furrows, cross structures, fragments stripping off, but still attached to the trunk, lenticels, spines and thorns or also fibrous and thick, smooth, corky, scaly, paper-like and flaky barks.

The two most exploited species of bark in the world

Typically Mediterranean, the cork-oak (Quercus suber) produces an abundance of external bark rich in suberin: cork. It is a naturally inert material, extremely insulating. The first harvest from the trunk, called 'male' cork, is of mediocre quality. It is mainly used for insulation panels. A decade later, the new-formed, 'female' cork, is of much better quality and much more homogeneous. It will be used to manufacture the well-known wine cork. The harvesting of the bark can be repeated a dozen times throughout the life of the oak.

From China to the Mediterranean, Egyptian papyri to the bible, the spicy bark of the cinnamon (Cinnamomum verum) has imbued for millennium the often-turbulent history of many civilizations...serving as perfume, medicine and spice. Harvest is in the rainy season, when the inner bark is enriched with flavor and can be more easily detached from the wood. Once dried, it will form cinnamon sticks. Ceylon cinnamon, with thin bark, a soft and subtle fragrance, is grown mainly in Sri Lanka.

Barks and their main uses

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 (Cinchona officinalis) 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 (Ficus grevei) on a stone, then add water to obtain a natural sun cream that protects their skin from sun burn.
- Baobab bark (Adansonia sp.) is sold as medicine to prevent calcium deficiency at the local markets of west Madagascar.

Other uses

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 more 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 bark of the canoe birch (Betula papyrifera) is a raw material used by American Indians to build their canoes, wigwams or to make containers and many handcrafted items.

- 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 common trees as lime (Tilia) have their barks very rich in fibers. After treatment, barks supply fibers used for making baskets, ropes, mats and even clothes.

Many natives of the Solomon Islands, central Africa or South America make actual plant cloth, called 'tapas'. It is mainly the bast fibers of the paper mulberry (Broussonetia papyrifera) or some local Ficus that are extracted from the bark and then beaten on a hard tree trunk until the desired thickness is obtained.

Other barks are used to make true sheets of paper. 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.

The use of tannins from the inner bark is an ancestral technique dating back at least 5000 years. They are particularly abundant in the bark of oak, acacia, birch, hemlock and mangrove trees. The chemical reaction between tannins and collagen from animal skins produce leather. Different barks can be used to add different colors or odors to the leather.

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