How does bark protect a tree

Tree bark facts and information

Bark is similar in many ways to our own skin and is essential for a tree’s survival. And a whole host of other species make use of this niche in the forest ecosystem. The visible part keeps moisture in and keeps infection out. But there’s a lot more to bark than first meets the eye.

What is bark?

Imagine for a moment you are wearing X-ray glasses and you can look just below the surface of a tree’s trunk. Here we see a layer of living tissue. Zooming in really close, this tissue is like a bundle of straws packed together. What we are seeing is the tree’s plumbing, conductive ‘pipes’ for transporting fluids. This tissue comes in two main forms.

The first layer we see is the phloem. Phloem is right below the surface bark and carries sugars from the leaves down to the rest of the tree. Further in is more ‘plumbing’ called the xylem or sapwood. These tubes carry water and minerals the opposite way, up to the leaves.

Sandwiched between these two layers is the cambium. The cambium’s job is to produce cells. On the inside it creates more xylem and on the outside it creates more phloem.

Xylem cells die quickly. They actually have to so they can fully play their role as pipes. After a while though, they become blocked and are replaced by newer xylem. This surrounds the old layer, which is why a tree’s girth expands each year. The blocked tubes become the tough heartwood of the tree. Heartwood gives the tree ‘backbone’ and is good at resisting rot and insect attack.

As phloem dies it is pressed outwards and becomes part of the bark. Many trees also have a cork cambium layer, outside the first one. Its job is to produce cork, which also forms a major part of bark.

So taking off our X-ray glasses, the main point is that everything outside the main cambium layer is the bark.

Bark as protection

The outer cork protects the tree from the elements – from scorching by the sun or drying by wind. It also helps to ward off fungal infection, insect attack, and the attention of hungry birds and mammals.

The bark of different trees has evolved to withstand the environment in which each species occurs. Scots pine bark offers protection from fire. In prehistoric times, wildfires would very occasionally sweep through areas of pine woodland. The thick, plated bark of Scots pines would help many of the older trees to survive.

The white bark of silver birch reflects sunlight and protects the tree from getting damaged by ultraviolet rays. Birch seeds can travel long distances and birch can easily find itself without the shelter of companions so this protection is important.

Many trees have chemicals within their bark that ward off fungi and insects. Scots pine has sticky resin and oak bark contains a lot of tannins, chemicals that taste off-putting and are also toxic in high doses.

Many trees including silver birch get rougher as they get older, which makes it harder for animals to damage the bark.

Bark as a habitat

Cracks in bark provide great habitat. The deep fissures and crevices in the bark of an old oak or Scots pine are a haven for many species of insects and spiders. These invertebrates attract birds such as treecreepers and crested tits.

Even after a tree has died, bark can be a home for all sorts of wildlife. Bats sometimes roost beneath loose bark and a multitude of invertebrates also live out their lives in this hidden world.

In the Caledonian Forest, some of the most obvious life on bark takes the form of lichens and small plants. Plants that live on trees, without actually causing them any harm, are called epiphytes. The texture of bark influences which epiphytes live upon it. In an old pinewood it is common to see many other plants such as blaeberry growing in the thick crevices of Scots pine bark.

The texture of bark, and thus the lichen communities, can change during the lifetime of a tree. Young hazel has fairly smooth bark, and so attracts lichens that prefer this texture, particularly the script lichens. (These lichens are distinguishable by the tiny ‘squiggles’ on their surface). Bark often gets rougher as the tree ages. It then becomes more suitable for other species, including the leafy, frogskin-like lungwort. The lichen community can also vary on different parts of the same tree. Aspen bark has smooth and rough areas, each supporting different species.

Chemistry can be as important as texture when it comes to bark as a habitat. Aspen bark is not as acidic as that of some other trees such as pine and birch. This means that it can support species of plants and lichen that might not otherwise be present in a pinewood. (Interestingly aspen can also photosynthesise through its bark!)

Food for wildlife

Bark does a great job of protecting the tree. Even so, there are some very determined creatures that are keen to get to the nutritious cambium, or the wood beneath it. Many mammals eat bark, and by looking at the height and details of the damage, we can find out what mammals are present in an area.

As their name suggests, bark beetles are among the insects that use bark. The larvae burrow down to get to the cambium and each beetle species makes distinctive galleries, or passages in the wood. They can then carry in fungal spores that the bark would usually repel, which is how Dutch elm disease is spread.

If bark is damaged around the circumference of the trunk, the tree is in real trouble. The phloem can no longer do its job of transporting sugars, and the tree may die. Voles often eat the bark at the base of young trees, killing young saplings. Deer also strip bark (as well as damaging it by ‘fraying’ their antlers on it to shed the velvet coating). The bark of trees including aspen and willow is an important food source for the European beaver.

While all this bark feeding can be destructive to individual trees, it is worth taking a step back. From an ecological perspective it shows how bark can support a wide range of different species. Also, when a tree is killed or harmed by bark damage, valuable dead wood habitat can be created for fungi, insects and many other organisms.

Bark’s main purpose is to protect the tree. But when we take a closer look we can see how every surface, nook and cranny in the woods can provide food and shelter for myriad living things. In this way bark also helps increase the biodiversity in a forest.

Sources and further reading

  • Brown, R.W., Lawrence, M.J. & Pope, J. (2004). Animals – Tracks, Trails and Signs. Hamlyn: London.
  • Mitchell, A. (1982). Trees of Britain and Northern Europe. Collins: London.
  • Steven, H.M. & Carlisle, A. (1959). The Native Pinewoods of Scotland. Oliver & Boyd: Edinburgh.
  • Street, L. & S. (2002) The importance of Aspens for lichen. In: Cosgrove, P & Amphlett, A. (eds.). The Biodiversity and Management of Aspen Woodlands: Proceedings of a one-day conference held in Kingussie, Scotland, on 25th May 2001. The Cairngorms Local Biodiversity Action Plan: Grantown-on Spey.
  • Tudge, C. (2005). The Secret Life of Trees. Penguin: London.
  • Wohlleben, P. (2017). The Hidden Life of Trees. William Collins: London.
  • (Accessed June 23rd 2020)
  • (Accessed June 23rd 2020)

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A Surprising and Diverse Reservoir for Water · Frontiers for Young Minds


Bark is the outside layer of wood that all trees have. Bark protects trees from harsh environmental conditions including weather, pests, disease, and damage from hungry animals. Just like leaves, bark is different across species. Some trees have thick, rough bark while others have thin, smooth bark. When it rains, bark acts like a sponge and absorbs water. Some trees have bark with large pore spaces that make it easy to absorb rain water quickly. Other trees have bark with smaller pore spaces, which absorb water slowly. Each tree species has a maximum storage capacity of water that can be held in the bark. In fact, some mature trees can store more than 100 L of water in their bark—that is about as much water as you would use in a 10-min shower! In this way, bark influences the water cycle of individual trees and entire forests.

The Water Cycle in the Forest

Have you ever thought about where rain goes when it falls from the sky? There are scientists called hydrologists who do just this. They study the water cycle as water moves through our environment, and this area of study is called hydrology. Rain is just one small part of the water cycle. The water cycle describes the movement of water on Earth and includes rain and snow, rivers and oceans, groundwater below our feet, and water that evaporates from soil, lakes, and leaves [1]. If we look closely at a forest, even more parts of the water cycle can be found. Some water lands on the leaves of trees and drops to the ground—this is called throughfall. Some water lands on leaves and flows down branches to the tree trunk and then to the soil—this is called stemflow. And some water lands on leaves and bark and is held there before being evaporated back into the atmosphere—this is called interception. Hydrologists care about how much water reaches the forest floor by throughfall and stemflow. We also care about how much water is “lost” to interception because that water cannot be used by the trees or animals in the forests, or by humans who might rely on the forests for drinking water.

What Is Bark’s Role in the Water Cycle?

Bark is the outside layer of wood that all trees have. Bark protects trees from harsh environmental conditions including weather, pests, disease, and physical damage from hungry animals. Bark is an important component of the water cycle because it acts like a sponge during storms. That means some of the rainwater is absorbed into the bark and does not make it to the forest floor. The amount of water that can be absorbed by bark depends on the tree species and its physical structure [2]. For example, if we compare the same amount (or volume) of bark among six different tree species, loblolly pine bark is about 18% solid bark and the rest of bark volume is open pore space (Figure 1A, far right). Pores inside bark are spaces that can be filled up by water during rain events. In contrast, mockernut and pignut hickory bark are about 38% solid bark and the rest is open pore space. So when it rains, loblolly pine has a lot more open pore space to absorb water!

  • Figure 1 - The barks of different tree species absorb different amounts of water.
  • (A) The barks of various tree species are shown in the top row. If we squeeze all the solid bark together and separate it from the open pore space, we can see that some species have more open pore space and some have less. (B) Some of this pore space is always occupied by hygroscopic water, which is water absorbed from the atmosphere, and whatever pore space is left over determines the amount of additional water the tree bark can store from rain storms.

How Do We Study Bark?

Measuring surface characteristics of bark is pretty easy. We can poke and prod the tree and measure how deep the bark is, down to the underlying wood. But sometimes, we need to take the bark off of the tree and bring it back to the laboratory to perform more experiments. Using chisels and hammers, we isolate a square patch of bark and gently pry the bark sample off of the tree. When we remove a piece of a tree to study it, we call this destructive sampling (Figure 2). Some of the experiments we might conduct on tree bark in the lab are listed in Table 1.

  • Figure 2 - Sometimes bark must be removed from trees so experiments can be performed on that bark in the lab.
  • This is called destructive sampling. In this picture, a piece of bark was pried from a loblolly pine tree in the Bankhead National Forest in northern Alabama. Don’t worry, the tree will be ok!
  • Table 1 - Experiments we conduct on tree bark to understand how water is stored.

Some Pores Hold More Water Than Others

Just like leaves, bark is very different depending on the tree species [3]. Some trees have thick, rough bark while others have thin, smooth bark (Figure 1A). Bark consists of two layers, the outer layer and the inner layer. The outer layer is composed of dead cells while the inner layer is living. Does bark with more pore space retain more water than bark with less pore space? Not exactly!

Loblolly pine has the most pore space but, despite this, it absorbs the lowest amount of water (Figure 1B). The pore space in post oak is very similar to sweetgum and white oak, but post oak can absorb much more water than the other two species because these barks have pores of different sizes. And even though mockernut hickory bark has much less pore space than loblolly pine, mockernut hickory can absorb a lot more water. These differences tell us that water absorption into the bark is a complex process and depends on many factors. We are trying to identify these factors through our research.

Some species have larger pores and some have smaller pores, and pore size also influences how much moisture the bark can absorb directly from the atmosphere, even when it is not raining! This characteristic is call hygroscopicity (Figure 1B). When we account for hygroscopicity, the pore space that can absorb additional rainwater diminishes and the capacity of our “sponge” is smaller. Bark hygroscopicity may reach up to 60% of the total amount of water absorbed by bark.


Now you know that bark is a reservoir for rainwater and that different tree species can have very different bark. Managing water resources is important because forests provide more than 75% of the world’s water for drinking and growing food [4]. If we want to be good managers of water resources [5], then we need to know exactly how much rainwater will make it to the forest floor and how much might be caught by leaves and absorbed into bark. By studying these topics, hydrologists help to understand the important role that trees and forests play in the water cycle.


This work is a contribution of the Forest and Wildlife Research Center and the Mississippi Agricultural and Forestry Experiment Station, Mississippi State University. This work was supported by the National Institution of Food and Agriculture, U.S. Department of Agriculture, Mclntire Stennis capacity grant # MISZ-032100. Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture.


Water Cycle: Movement of water between the Earth and the atmosphere.

Hydrology: The scientific study of water and its movement on Earth.

Throughfall: Rainwater that falls through forest canopies to the forest floor.

Stemflow: Rainwater that runs down tree trunks.

Interception: Rainwater that is captured by forest canopies and evaporated back to the atmosphere.

Pore Space: Open voids that can be filled with water or air.

Destructive Sampling: Removal of sample from the object of interest.

Hygroscopicity: The ability of a substance to absorb moisture from the environment.

Conflict of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Original Source Article

Ilek, A., Siegert, C.M., and Wade, A. 2021. Hygroscopic contributions to bark water storage and controls exerted by internal bark structure over water vapor absorption. Trees 35:831–43. doi: 10.1007/s00468-021-02084-0


[1] Weissinger, R. H., Thoma, D., and Biel, A. W. 2021. When nature gets thirsty. Front. Young Minds 9:610018. doi: 10.3389/frym.2021.610018

[2] Ilek, A., Siegert, C. M., Wade, A. 2021. Hygroscopic contributions to bark water storage and controls exerted by internal bark structure over water vapor absorption. Trees 35:831–43. doi: 10.1007/s00468-021-02084-0

[3] Arbor Day Foundation. What Tree Is That? Available online at:

[4] The World Bank. Why Forests Are Key to Climate, Water, Health, and Livelihoods. Available online at:

[5] Dobson, C., and Beck, G. 1999. Watersheds: A Practical Handbook for Healthy Water. Richmond Hill, ON: Firefly Books. 160 p.

Why do trees need bark?. Who is who in the natural world

Why do trees need bark?. Who's Who in the Natural World


Who is who in the natural world
Sitnikov Vitaly Pavlovich


Why do trees need bark?

The outer skin of a tree trunk or root is called bark. However, calling it a shell is wrong; rather, it is the outer part of the trunk or root. Sometimes it is quite difficult to determine its thickness, because in trees such as palm trees, for example, there is no boundary at all between bark and wood.

Why do trees need bark? One of its main functions is that it protects the inner, more delicate part of the tree. The bark not only prevents it from drying out, but also protects it from all kinds of external damage.

The bark of some trees is even capable of resisting flames. There are cases when in America huge mammoth trees were found with traces of a fire on a thick, fibrous bark, while the reliably protected wood turned out to be intact.

The process of bark formation can take years. For example, on a very young shoot of a maple, there is no hard bark as such, and small branches have an absolutely smooth surface. Therefore, when the woody layer of such a shoot begins to grow, the soft outer skin may burst in one place or another. However, such damage, as a rule, is not terrible for a tree and heals itself from the inside.

Gradually, one by one, sections of the outer shell of a young plant dry up and die; these dead scales are what give the bark its rough appearance. Some of them are constantly broken off or shed by the tree itself as it grows and matures.

People have learned to use the bark of many trees for their own purposes. For example, almost all corks are made from cork oak bark. The bark of the hemlock is used for tanning leather. The spice used in cooking, known to us as "cinnamon", is nothing more than the powdered bark of a tree growing in Southeast Asia. Quinine, a wonderful anti-malaria drug, is obtained from the bark of the cinchona tree. In medicine, in general, many substances are used from the bark of branches and roots of various tree species.

This text is an introductory fragment.

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Tree bark: structure, diseases, therapy

The tree is considered a source of strength. It is enough to hug him and stand a little, closing his eyes. But no tree will grow if its trunk is left unprotected. What is the bark of a tree called? It is rightly called the skin of a plant, which is a protective cover of the trunk. The bark of a tree occupies about a quarter of its total volume. It depends on the breed, age and growing conditions. The thicker the trunk, the more bark. In mature trees, its volume decreases. On the contrary, it increases if the growing conditions of the tree have deteriorated.

What is the protective layer of the barrel made of?

The bark of a tree is an important part of it. It protects the trunk from damage and harmful effects of the external environment, regulates the process of respiration and nutrition. Any, even the smallest, change in the surface of the bark can lead to the death of the entire tree if left untreated. The structure of the bark of a tree suggests the presence of inner and outer layers.

  • Inner layer - bast. It is represented by living cells, participates in the transport of nutrients from the crown to the roots of the tree and stores its reserve supply. The bast consists of three types of cells and tissues. The most important are the sieve elements. In coniferous trees, these are cells, and in deciduous trees, they are tubes.
  • Outer layer - cork. It's called a crust. The structure of the tree bark provides for a unicellular layer of living cells, which alternately divide in both directions, due to which the tree grows in thickness. The bark directly protects the trunk from environmental influences and consists of three layers. The middle layer of the tree bark contains a special substance - suberin. Thanks to him, its hydrophobicity is ensured.

Tree bark: species

The bark has protective, conductive, healing properties. And it complements the landscape of your site with texture, restrained colors and decorates it in the winter cold. Each tree is different and different: a unique pattern, a color that can be red, white, green, gray and orange, the nature of the surface. On this basis, the types of tree bark are:

  • Smooth.
  • Furrowed. These longitudinal and transverse stripes are especially clearly visible in oak and ash.
  • The scaly species of tree bark are easy to distinguish. The trunk is covered with scales that exfoliate well. A prominent representative is pine bark. Larch is covered with furrowed-scaly bark, which is formed by layering scales one on top of the other.
  • Fibrous. This type of bark is characterized by exfoliation of long longitudinal strips, like in juniper.
  • Warty. The bark of this species is characterized by small warts. A typical representative is the warty euonymus.

Diseases of the bark

Trees, like people, are susceptible to various diseases. From what do they arise? There are many reasons why trees get sick. An indicator of their state of health is the bark of a tree. She, like human skin, is very vulnerable. But, unfortunately, she is not able to take care of herself. The bark provides this care to a person, giving him a plentiful harvest in the future or delighting him with his appearance. The protective layer of the trunk is damaged by infectious diseases, pests, animals, frost, sunlight. And sometimes it simply does not keep up with the growth of the plant and cracks, forming deep wounds. Only good care and timely treatment will not allow the tree to die.

Black crayfish

Many tree bark diseases lead to their death. One such disease is black cancer. It begins with the appearance of sinking red-brown spots on the protective layer. The bark rises, breaks and cracks. Affected by black cancer, it is covered with small black tubercles. This is a parasitic fungus.

Often the bark falls off, forming open wounds. The disease develops gradually, affecting the trunk and branches, clasping them in a ring. Sick bark is an excellent place for the fungus to overwinter. Black cancer arises and develops due to burns, cracks and wounds. Weak development of trees accompanies the occurrence of this disease. Black cancer affects fruit trees at any age, but older plants are more vulnerable.


This disease most often affects old trees that are 20 years old or older. The infection penetrates under the bark of the trunk and branches due to wounds on it received from burns, frost, various pests and large animals. The bark of the tree is covered with a red-brown coating and becomes bumpy over time. Cytosporosis quickly spreads to healthy tissues. For one and a half to two months, the branches completely dry out. Over time, the tree will die if left untreated.


This tree disease is characterized by dark spots on the bark. Infected areas die, and depressions appear in place of the dead layer. A brown viscous liquid with a repulsive odor flows out of them. This is the dropsy cancer. Young trees die within one year, and old ones after a few years. If the disease has covered most of the bark, the tree can no longer be saved. To prevent the infection from spreading to other plants, they should be dug up and burned.

Infectious diseases and their treatment

How to treat tree bark from black cancer? First of all, the source of infection is eliminated. To do this, burn all the fallen leaves. In them, fungal spores live even in winter. When the annual pruning of diseased branches is carried out, the garden tool must be treated with blue vitriol so as not to cause infection.

If the bark of a tree is damaged by cytosporosis, it is necessary to remove the affected area and treat this place with copper sulphate. Then cover with var and bandage with a clean, dry cloth.

Circular lesion of the layer: how to treat

If the lesion of the bark has gone in a circle and involves the root collar, the tree may die. And if such a lesion is seen in the upper part of the trunk and branches, the tree has more chances for recovery. Wounds can be healed by grafting cuttings. If this does not help, you need to transplant the bark from a healthy tree. If the wounds are very small, you can wrap them with transparent polyethylene without covering them with pitch.

Lichens and their treatment

By the state of the bark on the trunk and branches of a tree, one can determine whether it is healthy or not. If the protective layer is covered with moss and lichen, there is a high probability of damage to the bark by fungal diseases and pests. Lichens tolerate frost and heat well. Spores of infectious diseases and larvae of various parasites coexist perfectly in them all year round.

How to treat tree bark if it is covered with lichen? Treatment should be carried out in spring or autumn in wet weather. To do this, with a hard nylon or metal brush, lichens are cleaned from the bark. First you need to lay burlap around the tree. After cleaning, all this is burned and buried deep in the ground. The cleaned bark and soil under the tree are sprayed with iron sulphate. You can wash the trunk and branches with a soapy-ash solution. Half a kilogram of ash, one and a half kilograms of lime are diluted in a bucket of water and insisted for several days. After spraying, the trunks and large branches of trees are whitened. Lichens begin to turn red and fall off.

Prevention of bark diseases

In order to prevent various diseases of the bark of trees, it is necessary to carry out regular prophylaxis. It is as follows:

  • The trunk and main branches are cleaned of old bark, which prevents the growth and thickening of the tree.
  • Mosses and lichens are removed.
  • Disinfection in progress. It is needed to destroy moss and lichen spores, pests and their larvae. Damaged tree bark is thoroughly washed with a soapy-ash solution. They also spray the crown, but the solution is diluted with water several times. You can wash the barrel with copper sulphate by dissolving 100-200 g in a bucket of water. In its absence, iron sulfate is used. But it needs more per bucket of water, 600-800 g. Gardeners often use oxalic leaves for disinfection. To do this, on the bark, you need to remove all the growths to the very wood, level the wounds along the edges and rub with a sorrel leaf. They will quickly tighten with a new protective layer.
  • Cracks after disinfection should be covered with pitch or a mixture of clay and lime. If nothing, just whiten.

Often hollows can be seen on the trunk and branches. They eventually lead to the death of trees due to developing infection. They definitely need to be sealed. To begin with, garbage is removed from the hollow, the bark and wood are cleaned from rot. Then disinfection is carried out with iron sulphate. After that, the hollow is sealed with pieces of cork or a mixture of lime with cement and sand. If the hollow is very large, it is clogged with stones, rubble, bricks and poured with cement mortar.

Thermal damage

Trees are subjected to rapid temperature changes during the growth process, when the bark is strongly heated by the sun during the day and cooled at night. This leads to the formation of frost holes, cracking and sunburn. Thermal damage is dangerous because it causes partial or complete death of the bark, which occurs due to blockage of the vessels through which nutrients move. This disease is called necrosis and is characterized by the sinking of the affected tissues. Frost crackers can be easily identified by the bark separated from the trunk, where pests and all kinds of fungi settle and breed. If frost holes are not identified and neutralized in time, hollows may form.

Diseases of the bark of trees can be caused by the sun's rays, when their direct contact leads to burns. This usually happens with the onset of spring, when the daytime air temperature becomes positive, and the night temperature becomes a big minus. There is a cooling of the internal and external parts of the tree. As they cool, they shrink. Moreover, the outer parts are faster than the inner ones. As a result of this, a rupture of the cortex occurs. To prevent it, the trunks and branches of trees are whitened and tied with burlap before the onset of winter cold.

Prevention of thermal damage to the bark