Bark Inclusion

If you happen to be a tree, one of the bigger challenges you have is this: I need energy to grow and reproduce! So I’m gonna reach for sunlight, water, and nutrients. But how far and fast can I reach before something like this happens?

The photograph above shows an American elm (Ulmus americana ‘Princeton’) planted years ago by Tree Watch and Trees Atlanta in Freedom Park.

American elm is really great at reaching. It can grow fast. When it’s at home — ‘home’ being rich, alluvial soil in a forested floodplain along a river or stream — a tall American elm will have channeled so much of its energy into growing upward, in order to reach that lovely sunlight at the top of the forest, that it may have no branches at all until the 50- or 60-foot mark.

Photo credit: Christian O. Marks

Of course, an American elm growing in an open, grassy section of Freedom Park has no difficulty at all accessing sunlight. It’s like a kid left alone in a candy shop. (Think of that poor elm tree in Freedom Park as a kid with a terrible tummy ache.)

Amid an open-grown elm’s profusion of multiple branches, it can happen that bark may get trapped between segments of the tree. Here’s an example of bark trapped in this way (in Nothofagus alpina):

Photo credit: Duncan Slater

And here’s what it looks like inside such a tree. Look for that dark vertical line:

Photo credit: M. D. Vaden

We call this “bark inclusion.”

Can you guess what the problem is when bark is “included” between two stems or a stem and a branch? Bark, remember, is not living tissue. Right: it’s a weak point in the overall structure of a tree.

Not every junction between stems or between a stem and a branch, even a “tight” junction, has this included bark. In a properly formed junction, you will see what is called a “bark ridge” on the upper side of the union (on the left, between two trunks; on the right, between a trunk and a branch):

Here’s another photo of clearly visible bark ridges on either side of a tree:

Below these bark ridges lies specialized wood that is strong enough, in most cases, to help the tree “hang” together. You can see it below. Note the way the grain of this strong wood is in whorls.

Photo credit: Duncan Slater

When included bark develops between parts of a tree, however, there is no such extra-strong wood.

Why does bark sometimes become included? Interestingly, a researcher named Duncan Slater’s theory is that a relative lack of what he calls “dynamic and static loading” is a major contributor. (The term apparently is “biomechanics.”)

In effect, it’s too little stress or strain upon the tree from gravity and wind. Stress, at the proper level, supports a tree’s normal development. Too little stress — for example, not much movement in the wind — can result in abnormal development.

Isn’t that fascinating? We think of trees as stationary. They’re not, or they aren’t supposed to be.

So one theory for why our Freedom Park elm developed included bark and split in two is that the different stems and branches of the tree did not move independently in the wind. So many of the parts of the tree were roughly equal in size and growing so close together that they tended to move in unison, when each part needed to flex independently.

All of that speaks to the need for proper pruning beginning early and continuing until the tree has a sustainable structure. (Having said that, your Tree Watch chairman is hanging his head in shame.)

4 thoughts on “Bark Inclusion”

    1. Ken, thanks for the comment. The Princeton American elm is a cultivated variety of the species, selected from a local tree by a Princeton NJ nurseryman back in about 1920. Not a hybrid like, say, a cross between an American chestnut and a Chinese chestnut.

      Casey Trees in DC has this comment on hybridization of elms: “There is a good reason why American elms are cultivated carefully from single specimens rather than hybridized .. [It’s] because it is a tetraploid — that is, it has a chromosome number twice that of all other elms (56 versus 28). Therefore, most hybrid breeding and selection work does not include American elms. Thousands of attempts to cross the American with the Siberian elm have failed.”

      The horticulturalist back in the twenties just liked the look of an elm he saw, and so he propagated it by taking a cutting and grafting it. (Later his American elm tree demonstrated itself to be resistant to Dutch Elm Disease. So it eventually became a great seller.) But the feature of the tree he fell in love with, its very upright form, turned out to be a problem. It’s almost TOO upright. The junctions between the branches and the trunk are so, so narrow. And that contributes to the frequency of bark inclusion.

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