Giving a tree talk a few years ago, I used a fishing rod to explain something about tree structure. My wife, Stephnia, said it was an excellent idea, so I will revisit it.
The shaft of a fishing rod is tapered and flexible. The end you hold is thick and the other end is very thin. When loaded with a fish on the line, the thin end can bend dramatically while the thick portion bends less.
The taper and flexibility relieve some of the load and transfer it down to the thicker end anchored in your hand.
The same principle is at work in trees. It is pretty obvious when you look closely at an “excurrent” — single trunk, straight — tree like a coast redwood.
Watching it in strong winds, you see the upper portion bend and sway while the base stands steady. The same applies to “decurrent” — branched trees with multiple limbs — but it is not quite as visually obvious.
To my mind, there is a sort of miracle of nature at work in this.
It just so happens that the older parts of the tree grow thicker with age while the newer portions — the growing tips — are thin. Voila, taper! An ideal engineering principle naturally emerges in a tree as it grows taller.
It reminds me of something the great composer, Leonard Bernstein said of the music of Beethoven: It has a sense of inevitability.
Or as the astronomer and cosmologist Alan Sandage said of the secret of “why there is something instead of nothing.”
“When we eventually discover it,” he said, “we will probably look at it and say: Of, course! How could it be any other way?”
This brings to mind the principle of elegance in the sense of an idea or object that shows compelling simplicity and ingenuity. Trees show elegant problem solving in many ways.
Look again at a tree in the wind. The tapered branches are of differing lengths and exposed to the wind at differing angles. As a result, they move chaotically in gusts of wind, redistributing and shedding the wind load rather than sending it entirely and simultaneously to the lower trunk and anchoring roots. The principle is called “mass damping.”
Along with that, the flexible wind-blown branches bump into each other, limiting their motion, helping them to avoid overloading and structural failure. The principle is “buffering.”
Leaves demonstrate engineering principles as well. To accommodate forces of weight and wind, the petioles — the little stem that attaches the leaf to the tree — bends and twists. Leaves of many broad-leaved species bend, fold and curl into streamlined shapes, which reduces the load on the petioles and stems. Part of the reason palms withstand hurricanes is the ability of the fronds to fold up and shed the wind.
In addition to natural taper resulting from the new stem being thin as the older stem increases in girth, trees show “Response Growth.” A heavily loaded stem can add wood, thereby reinforcing the stem. It is visible in the muscular look of old tree trunks and limbs.
Look closely at the trunk of a healthy, old oak and you might see swellings and light-colored tissue in fissures in the outer bark, where the tree is responding to the weight of the canopy.
You see it in the cross section of heavy limbs of conifers: The “center” is not at the physical center of the limb. The wood is thinner above and much thicker below, where the tree has laid down compression wood on the underside in response to the load of the long limb.
It serves us well to understand and work with these inherent properties of trees nature has worked out over vast amounts of time.