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Bill Pramuk

Bill Pramuk

After the power went out about 7 a.m. last Sunday, I was standing at the kitchen window, in the early morning light, watching a palm and a redwood across the street, moving in the gusty wind.

One is a Mexican fan palm (Washingtonia robusta), about 80 feet tall. There are many that size in the neighborhood, probably all planted in the late 1960s when the homes were built. The other is a coast redwood (Sequoia sempervirens), about the same height. We have lots of those too, the old-fashioned wild seedling redwoods, available in the nursery trade before the cultivated varieties were introduced in the '70s.

I could see a sharp contrast in the way the two responded to the wind. The palm trunk swayed over about 20 degrees and the fronds folded in. The redwood trunk stood relatively still, while the branches jostled chaotically.

I think of the palm as a sort of giant grass. They are close relatives. Consider how grasses wave in the wind, like the “amber waves of grain.” The stems give and bend, while the roots hold tight in the soil. The huge palms have developed the ability to bend, fold up and let the wind slip by. In addition to flexibility, they have a strong anchorage of numerous, finely divided roots. And since their above-ground structure consists of a single stem, with just a top-knot of fronds, they have no wide-spreading, cantilevered limbs.

It is not only the lack of wide-extended weight that helps palms resist structural failure. There is also a lack of torsion force. Tree stems fail more often from twisting force than from direct bending. That’s because tree stems have great tensile strength but relatively weak resistance to torsion. In a palm, the wind finds nothing grab and twist.

Palms are “built” to withstand severe winds. Thinking back, I do not recall any local instance of a palm trunk breaking except for one. That was a very tall Mexican fan, about 100 feet tall, which grew by Jefferson Street near Napa Creek. It exploded when it was struck by lightning.

In contrast, coast redwoods do not show the same kind of bending in strong winds. The taper of the trunk to a small diameter top does allow for bending, like a fishing rod, which transfers the force downward to the larger diameter trunk and into the roots and soil, but a great deal of the wind force is dissipated by the chaotic motion of the branches. Two mechanisms are at work here: “mass damping” and “buffering.”

Mass damping occurs when branches of different lengths and different positions move at different times in response to wind gusts. The chaotic motion dissipates the force. In effect, the wind cannot make the whole tree, or all of the branches, get moving in unison, so the net effect is less force on the tree at any given time.

Buffering occurs when adjacent branches bump into each other during wind gusts. The neighboring branches limit each other’s’ motion. The net effect is fewer branch failures. That is why I no longer favor pruning to thin redwood trees to “let the wind move more freely through the tree,” unless there is some structural concern with the condition of the trunk or weakly attached multiple tops.

Trees have wonderful ways of responding to the environment. One is (and I love the opportunity to use this word) “thigmomorphogenesis,” the response of plants to mechanical sensation by altering their growth patterns. This response to touch can result from wind, rain, and even from rubbing by passing animals.

Windblown trees of a given species grow relatively short and stout, compared to trees of the same species sheltered in a forest.

Early Monday morning, out walking our dog, we saw Halloween decorations blown out their front yards, a plastic skull in a gutter, mixed tree debris, weakly attached limb failures, and a small crape myrtle blown over along with its two tree stakes — the aftermath of a very windy day. But the redwood and the palm were standing straight and unharmed, having coped with the wind in their individual ways.

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Bill Pramuk is a registered consulting arborist. Visit his website billpramuk.com, email questions to info@billpramuk.com or call him at 707-363-0114.

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