Why Do Trees Have Bark?

manzanita (Arctostaphylos sp.) with smooth, cinnamon bark

It happened that a certain student, full of philosophical curiosity, went to a sage, someone steeped in ancient wisdom, and complained, “I don’t understand,” the student began. “Why did God create a world where money is a necessity of life?”  The sage paused for a moment. “The real question,” he finally answered, “is ‘why did God created a world where food is a necessity of life?’.”  Unlike the angels, I think the sage was saying, we are not purely spiritual creatures, but have an inescapably physical side as well.

I thought of this exchange between student and sage the other day when my wife, who teaches third grade, brought me a research topic from her class worksheet on trees.  “Very few trees have smooth bark,” I read.  “Find out why most bark is rough and has scales or cracks.” But perhaps the real question that should be asked is why bark, whether rough or smooth, is a necessity of arboreal life in the first place?
Origin of Word Bark Explains Why Trees Have Bark
The answer may be found in the origin of the word bark, which is the same as the origin of the word birch.  Birch, that familiar tree with white and gleaming bark, comes from a Nordic word for — wouldn’t you know it — white and gleaming.  In other words,  birch bark, whose texture resembles that of human skin, is the paradigm for bark in general.  And just as our skin provides protection from physical blows, from temperature extremes, from fungal and bacterial disease agents, from UV radiation and from harmful chemicals, bark serves a similar protective function in trees.  Trees, like humans, have a strong tendency to want to grow up vertically and even reach towards heaven but, alas, are forever bound to the earth and, like us, require protection from, in Shakespeare’s words, “the thousand natural shocks that flesh is heir to.” (Hamlet, Act III, Scene 1).
However, I do take issue with the premise of the third grade investigation mentioned above.  I was rather surprised with the assertion that “most bark is rough.”  If you look at North American trees, and temperate zone tees in general, this statement may be true.  Yet, once you leave the temperate zones and enter the tropics, nearly every tree has smooth bark. Think of ficus and citrus trees, for example, which are indigenous to the tropics and have smooth bark.  When you consider that the number of plant species in the tropics is significantly greater than the number of plant species in temperate zones, you could easily conclude that more trees have smooth than rough bark.
Why Some Trees Have Rough Bark and Some Trees Have Smooth Bark
In truth, there are advantages to rough bark and there are other advantages to smooth bark, too. Rough bark is best suited to withstand harsh changes in weather from one season to the next such as the freeze-thaw cycle from winter to spring.  Rough bark is also better able to protect from fires than smooth bark.  But smooth bark also has significant advantages, particularly in regards to warding off attack of insect pests.  In the tropics, where warm and moist conditions prevail  practically year around, promoting near constant insect activity, smooth trunks are an asset to trees since it is more difficult for insects to gain a foothold on smooth trunks.
A certain species of pine tree growing in the Rocky Mountains was evaluated for the presence of bark beetles, the leading insect pest, in general, where all pine trees are concerned.  It was found that young trees with smooth park were less infested with bark beetles than older trees with rough bark.  On the same tree, or even on the same branch, where both rough and smooth bark were found, trunk or stem sections with rough bark were more likely to be attacked by bark beetles than sections with smooth bark.
Keep in mind that several popular smooth barked trees, such as crepe myrtle (Lagerstroemia indica), lemon-scented gum (Eucalyptus citriodora), and California sycamore (Platanus racemosa), are constantly exfoliating or sloughing off old bark.  This is another defense mechanism utilized by smooth barked trees to shake free from insect pests before they can gain a foothold, lay eggs, and emerge as larvae to bore into the trunks of these trees.
Rough and Smooth Bark Trees May Both Live for more than 2000 Years
Bark texture, incidentally, provides no clue as to longevity of tree species.  Redwood bark is significantly furrowed while olive bark is smooth.  Specimens of both redwood and olive trees may be found growing in California (in the case of redwoods) and in Greece and Israel (in the case of olives) which have been alive for two thousand years or more.
Tip of the Week:  When considering garden design, having an eye for attractive bark is useful when taking the appearance of your garden in late fall and winter into account.
In general, tree bark is most visible and stunning on wet and overcast days, even on trees that don’t lose their leaves. Smooth cinnamon red bark is especially prized and may be found on two native trees that are grown in our area. The most famous is an arboreal manzanita (Arctostaphylos manzanita), which can reach a height of 20 feet. You must be patient with this tree and give it less than full day sun for best results.  You can find manzanita growing wild on the slopes that overlook Castaic Lake near Lake Hughes Road. A related native with similar bark is the madrone (Arbutus Menziesii). Both the manzanita and the madrone have bell-shaped flowers that bloom in winter. California ironwood (Lyonothamnus asplenifolia) has rough textured bark of the same color.
In any season, no tree can match the dignity of the California sycamore. A month or some from now, once its leaves have fallen from its limbs, nothing can compare to the California sycamore as an embodiment of the bare-bones beauty of winter. Its mottled bark and sculptured branches would make it the piece de resistance in a display of winter-dormant arboreal masterpieces.
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