All About Bark

rough and smooth bark


Anyone who regularly reads this blog knows that I am in love with trees.  They are marvelous beings in a great many ways.  And most recently, I have been musing on a particular aspect of trees–their bark.

I wonder why there are so many different types of bark.  Why do some trees, like the beech, have smooth bark?  Why do others, like oaks and hickories, have thick, furrowed bark?

Then there is color.  Why are most trees some shade of brown or grey?  And why do some trees have bark of a different color, like the white of a birch or the green of a palo verde?

Of course the overall answer is that the 60,000+ tree species in the world have all evolved to adapt to particular, local environmental conditions.

But I am curious to know more about the differences among trees’ bark, and how the bark of each species is uniquely suited to help that species thrive. So I decided to do a little research, and it turns out that bark is as incredible as the rest of the tree!

All Bark Does It

Despite its many different looks, bark serves  the same essential functions for all trees.

Bark protects a tree’s internal living systems from the outside world–from extremes in temperature, from loss of moisture, from fire and rainstorms, from disease, and from insects that could damage or even kill a tree.

Bark is more than what we see.  Just inside a tree’s outermost surface is a layer of inner bark that also plays an essential role.  Called the phloem, the inner bark circulates nourishment throughout the tree.  This ‘food’ is the product of photosynthesis–that process by which leaves take in sunlight and carbon dioxide and transform them into sugars that feed the tree.

The Smooth and Rough of Bark

All young trees have smooth bark.  And some species keep that smooth bark their entire lives.  Other species, however, develop rough, furrowed bark with age.  As a tree grows–as that thin layer called the cambium just inside the inner bark adds living wood to the tree every year (think tree rings)– pressure is exerted outward against the bark.  And it is how the bark responds to this pressure that determines whether the outer bark will remain smooth or will become rough and furrowed.

For example, beech bark responds to the internal pressure of growth by expanding.  The bark of a beech tree grows slowly to accommodate the new, growing wood inside.  At the end of a beech tree’s life, it has the very same bark it started out with, only more of it.

But rough-barked trees, like oaks and hickories, respond to pressure from the growth taking place inside by splitting into vertical pieces.  And as the outer bark splits into sections, a new layer of bark forms inside.  This process is repeated again and again as the tree continues to grow, resulting in the rough, deeply furrowed bark of older trees.

Smooth vs Rough: Different Benefits

Smooth bark can help a tree with photosynthesis.  While photosynthesis is typically the leaves’ job, smooth bark is dotted with small openings that take in carbon dioxide from the air to generate additional food for the tree.  In times of defoliation, due to disease or drought, this added photosynthesis can help a tree stay healthy.  Bark photosynthesis tends to peak in early spring and late fall, when there are no leaves shading the bark.

If you’ve ever wondered about the dark, horizontal lines on birch bark, or the diamond-shaped marks on an aspen trunk, or the raised, white dots on spicebush bark, these are all lenticels, openings that allow carbon dioxide to enter the tree.

Rough bark offers a different kind of protection.  Trees with rough bark are much better defended against fire than are trees with smooth bark. All over the world, rough-barked trees in especially fire-prone areas develop even thicker bark than they do in less fire-prone regions.  The bark of Scots pines and black oaks, for example, are extra thick in regions where fire occurs frequently.

And the ridged, furrowed bark’s greater surface area contributes to an even temperature inside the tree.  The thickness of rough, furrowed bark also provides protection against injury from outside.

Bark Color

Most bark is some shade of brown or grey, which experts say is largely because the many chemical substances in the bark, when combined, create a dark color.  Think of the brownish color that results when different colors of paint are spread over one another.

But what about trees with white or green bark?

The white bark of the birch reflects the sun’s rays.  While one might think that a tree of the cold north, like the birch, would do better to absorb, rather than reflect, the sun’s warming rays, that actually is not the case.  If birch bark were to absorb the sun’s rays on a sunny winter day, that warming, followed by the extreme cold of the northern winter night, could cause rapid fluctuations in the temperature of the birch’s cambium.  Such fluctuations between warm and very cold could result in cell death and severe injury to the tree.  So white birch bark serves a protective role.

birch bark 

And what about the greenish color of the palo verde? Unlike the bark of many smooth barked trees that plays a secondary role in photosynthesis, the bark of the palo verde is the tree’s primary source of photosynthesis.  Palo verde bark is filled with chlorophyll, which makes it green.  Photosynthesis through the palo verde’s bark accounts for two-third’s of the tree’s photosynthesis, with the leaves playing a much lesser role in the process.

palo verde


Other Protective Features of Bark 

Certain trees, smooth and rough barked-trees alike, ‘exfoliate’ or shed bark.  Consider the peeling birch.  Or the shagbark hickory with its long, vertical strips of bark that seem to hang loosely from the trunk.  Or the sycamore that sheds hunks of curled bark.


This exfoliation may serve different functions for different trees.   The horizontal curls visible on a peeling birch prevent mosses and lichens from establishing themselves on the bark, preventing the clogging of lenticels, which would interfere with photosynthesis.

But when it comes to how the shagbark hickory and sycamore benefit from exfoliation, I could find no solid explanation, only speculation.  Tree experts believe that exfoliating may rid trees of aphids, harmful fungus, and bacteria.

Bark’s chemical composition often plays a protective role too.  Birch bark, for example, is high in volatile oils.  These oils make birch bark so waterproof and resistant to decay that tubes of old birch bark can sometimes be found on the forest floor, even after the wood inside has rotted.  And the bark of black cherry exudes a bitter almond scent which deters browsing animals that might damage the tree.  Oak bark contains tannin, an astringent substance that is toxic to insects.

Barking Up the Right Tree

Tree bark is truly incredible.  Just like trees themselves.–April Moore





5 Responses to “All About Bark”

  1. James Says:

    Before we covered the earth with asphalt, concrete and skyscrapers, Native Americans were discovering the benefits trees had, to aid in their survival.

  2. Virginia Seiser Says:

    A novel for tree lovers: “Overstory” by Richard Powers. Individuals from very different backgrounds come together to protect old growth forests.

  3. Tony Dorrell Says:

    Thank you April for this very interesting and educational message about tree bark.

    As a professional forester I share your love for trees, and appreciate your enthusiasm for spreading the word about their positive contribution to people and our environment.

  4. admin Says:

    I appreciate these comments. And Tony, I share your long-held belief that, “TREES ARE THE ANSWER!”

    Ginny, thanks for mentioning THE OVERSTORY. I just finished reading it, and I loved it.

  5. Laura Crites Says:

    Thanks so much April. I share your love for trees. As I look out my window this morning at the forest I feel the presence of spirit so profoundly that I am moved to tears

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