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Creeping Soil, Bending Trees

A shot from the ground of a stand of bent trees on a slope
Alikaj2582/Getty Images/iStockphoto
Magical crooked trees in the forest. Albania, Europe

As a rule, trees tend to grow straight. Vertical. This is especially true when there is competition for light. It gets crowded in the forest, and light is a limited resource. So it’s not the best strategy to take your time getting up into the canopy, and into the sun. But despite all this, something I see every time I hike the mountains around Gallatin Valley is trees that do not grow straight. Trees with trunks that curve in one direction before eventually straightening out.

Now I don’t want you to picture trees growing at weird angles on flat ground – this always occurs on a slope, and the outward side of the bend always angles downhill. The tree is growing upward – but not straight. So why not just grow straight up from the start? Why the bend?

Turns out the answer is actually in the soil. Or more accurately, the movement of the soil. "Soil creep" is the gradual movement of the substrate downhill over time. It’s a slow process, but all the soil on a slope is moving. Importantly for trees, the soil moves more quickly on the surface than deeper down. Because of this, there’s more soil pushing the tree at ground level than below – and gradually, eventually, this tilts the tree.

To compensate, trees will put on thicker growth – wider rings – on the downhill side, slowly changing the angle of the trunk to regain an upright posture. Eventually, the roots grow deep enough and strong enough that the soil can no longer tilt the tree. At this point, the growth will even out to correct the angle, and the curve straightens! You can even find trees that apparently overcorrected, and you get what looks like a very stretched-out S-shape before the tree goes fully vertical.

The phenomenon itself is fascinating, but what keeps me looking are the differences between trees. Sometimes the curves are gentle and gradual, affecting a large proportion of the tree’s total height. This would mean that the speed that the soil was moving downhill was fairly steady for the time it took the tree to get big enough to resist the push. Sometimes the bends are sharp, with the trunk straightening out after only a few feet. This indicates that the tree was affected very strongly by creep as a sapling, maybe by a much looser, wetter soil, or even a small landslide. Sometimes younger trees have a deeper bend than older trees, or vice versa, indicating that the younger trees are dealing with a different rate of movement than the older ones did in their youth. The bends in the trees tell the story of the soil – how it has moved, continues to move, and how this movement has changed over time.

Sometimes the trees in an area may bend different directions, and clue you in to a hidden channel or the bed of a seasonal stream. The direction of the bend will always point downhill – or more accurately, where downhill was when the tree was small. One of the strangest trees I’ve seen had bends pointing in two different directions – one that was the current downhill, and one that was off by about 90 degrees! Avalanche? Super fast erosion? I can make guesses, but that one is beyond me.

Soil creep can affect other things too – the tilting of tombstones and telephone poles, the gradual encroachment of earth onto roads and sidewalks, and even the smoothing and rounding of hills. Of the two factors that cause soil creep, one is very common (that being elevated dirt), and the other is a universal law of physics (that being gravity). So it is truly happening everywhere, all the time. And the effects of it are everywhere, too. If you haven’t noticed the bent trees, keep an eye out the next time you’re in the hills. It’s the most obvious sign telling us that the soil is not static, even if it seems so to us, with our impatient animal eyes and short animal lifespans. Take a note from the trees. Even “solid ground” is more dynamic than it seems.

I’m Cedar Mathers-Winn for Field Notes, brought to you by the Montana Natural History Center, providing natural history education for schools and the public throughout Montana. For information on upcoming events and programs at the Center, call 406.327.0405 or visit our website at

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