Why do trees fall over in a storm?

The more accurate version of this question would really be something like, "Why do some trees fall over in a storm while others stay standing?" The answer is more complex than a simple distinction between old, rotted, and weak vs. young, healthy, and strong. Instead, writes Mary Knudson at Scientific American blogs, trees fall because of their size, their species, and even the history of the human communities around them.

“Trees most at risk are those whose environment has recently changed (say in the last 5 – 10 years),” Smith says. When trees that were living in the midst of a forest lose the protection of a rim of trees and become stand-alones in new housing lots or become the edge trees of the forest, they are made more vulnerable to strong weather elements such as wind.

They also lose the physical protection of surrounding trees that had kept them from bending very far and breaking. Land clearing may wound a tree’s trunk or roots, “providing an opportunity for infection by wood decay fungi. Decay usually proceeds slowly, but can be significant 5-10 years after basal or root injury.” What humans do to the ground around trees — compacting soil, changing gradation and drainage “can kill roots and increase infection,” Smith warns.

Read the full piece at Scientific American Blogs

Image: West Philly Storm - Trees Down, a Creative Commons Attribution (2.0) image from kwbridge's photostream


  1. I do a lot of horse riding in the woods, and often I see trees, surrounded by other trees, that just fell, after a storm, while the other trees around it just stand.

  2. After last year’s LA windstorm, the county tree guy was on KPCC and mentioned over-pruning as a culprit. I guess trees are able to grow to properly balance themselves out, but then people trim off branches that look aesthetically wrong or “hack” tree trimmers convince tree owners to remove those branches.

    1. That seems very wrong. Trees go over partly because they’re top-heavy. A well pruned tree should be less likely to blow over.

    2. My understand was that the tree damage from that storm was so severe because the wind direction was unprecedented, so trees hadn’t grown to support themselves in that direction, and trees that were weak in that direction hadn’t been routinely tested.

      “Over pruning” can mean lots of things.  You can hack one side of a tree and leave it unbalanced, but more likely he was referring to “topping” which then causes particularly weak and poorly attached “stress” growth, and that, decades down the line, can leave huge trunks of the tree ready to split off easily.

      A trained aesthetic pruner will never prune a tree in such a way as to compromise it’s structural integrity, and only certified arborists should ever prune trees that are large enough to hurt people.

      L.A. specific information on Urban Forestry.

  3. My new neighbors decided to “save 20%” by using a no-name guy with borrowed equipment instead of the licensed, experienced arborist that everyone else in the area uses.  The arborist recommended only pruning a few of the other trees but then taking down the one large tree that is half dead and hanging over all our power lines.  End result?  All the healthy, shorter trees are gone: apparently they were easier to chop down than prune.  And that half-dead tree?  Yeah, that’s still there, now with NO other trees around it as wind break or other protection.

    People are idiots sometimes.

  4. Trees also fall down because of the nature substrate they happen to be growing on and it ain’t all about what humans do. Though goodness knows, humans do and do and do a lot to trees and everything else. We are busy apes. Trees in wetlands and along stream and river banks are very vulnerable when the soil gets saturated and the roots can’t hold.

    But in a proper forest, it’s okay if a lot of trees fall. There are plenty of organisms that get busy when a tree falls. It is more of an issue when you have phone or power lines under the tree, or if you have cut down most of the trees in the area to put up your ugly Toll Brothers mini mansion and then you are weeping over the two trees you’ve allowed to grow fall down because it throws your landscaping all to hell. :P

  5. As Pipenta mentions above, the soil that the trees grow on, and what lies underneath that soil, make a huge difference. I lived in Memphis for a while, and wind storms would blow over some old, venerable trees because even though the trees themselves looked strong with thick trunks, they were growing on a relatively thin layer of soil over clay, and the root system was almost all near the surface.

    You also had some trees that were off-balance because of incredibly heavy-handed pruning by the local power company; the utility would prune trees near power lines, starting at the river and working their way east, and since they may not have gotten to a particular tree in a few years, they’d chop out a big arc around the power line, leaving an ugly scoop cut out of the crown. (I think that this may have had the paradoxical effect of weakening the tree sufficiently so that it was more likely to blow over in a storm and knock the power line out.)

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