It sounds like science fiction, but it is not. Every spring and during particular summer weather, limbs can fall from trees without warning, and seemingly for no reason at all. It happens when least expected, while the weather is warm and perhaps humid, but notably without wind. The lack of wind is what makes it so unexpected. It is a phenomenon known simply as spontaneous limb failure.
Those who witness it might think that the arborists they call to clean up the mess will not believe their descriptions of what happened. Yet, arborists are familiar with it. Quite a few species of trees are notorious for it, especially in urban landscapes where they get watered regularly. Most of such trees are either chaparral trees that do not expect much water, or riparian trees that do expect it.
Spontaneous limb failure occurs as warmth accelerates vascular activity, but humidity inhibits evapotranspiration, which is evaporation from the foliage. Accelerated vascular activity increases the weight of the foliage. Inhibition of evapotranspiration limits the ability of the foliage to eliminate some of the excess weight. Limbs break if unable to support the increasing weight of the foliage.
Spontaneous limb failure is not as easy to predict as the more familiar sort of limb failure that is caused by wind. Limbs that get blown down typically exhibit some sort of structural deficiency or disfigurement prior to failure. Some limbs that succumb to spontaneous limb failure do so as well, but most do not. They just happen to be the healthiest and most densely foliated parts of a tree.
Native coast live oak and valley oak are the two most familiar of the chaparral trees that are notorious for spontaneous limb failure. Native cottonwood, willow, box elder and sycamore are riparian trees that are perhaps even more susceptible to spontaneous limb failure. Sweetgum, carob, stone pine and various eucalypti are some of the exotic trees that might drop limbs spontaneously.
In summer, spontaneous limb failure is less likely as growth slows and limbs strengthen.
Tony Tomeo 
Interesting
I never knew why trees sometimes just drop a limb. Thanks for explaining
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That is why it is such a surprise to those who witness it. It is very different from a limb just getting blown down.
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I totally agree. I knew that this happened–here we sometimes say that oaks are self pruning or self cleaning– but that didn’t explain all the other trees. Great explanation.
Karla
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Thank you.
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Fascinating. When I saw the title, I briefly thought it was about human limbs, which would be bad for a working gardener.
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Oh, I didn’t think of how it could be misconstrued.
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During the last half dozen years or so, there have been several incidents in the LA area, in which limbs have fallen unexpectedly, often in parks (reported because that’s where people could/have been underneath), and often in late afternoon when the weather has been quite warm and dry. There has never been any explanation of these incidents other than that arborists were looking into the affected trees to determine what happened. Are there any preventive measures that can be taken?
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Prevention is not something that is considered. The trees that are most susceptible are the healthiest sort. We arborists are not often concerned with the healthy trees, so potential for spontaneous limb failure typically gets ignored. I sometimes notice it about to happen in sweetgum trees, and am able to prune to eliminate excessive weight before it happens, but I can not prevent all of it.
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How interesting that the most susceptible trees are the healthiest. And how sad that they suddenly lose limbs. Thanks for a good explanation!
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Distressed trees do not weigh as much, and are not as vascularly active.
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Thanks Tony, very succinct article – this happens often here with Eucalyptus too – they look so nice for camping under…. but its not a good idea.
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You are welcome.
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Great information! The live oaks down here don’t seem to have a top size and there are plenty of older ones that have evidence of past large limb breakage. Now I know it may have been rain, humidity and the resulting transpiration rates that cause limb failure, very interesting! Thanks!
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Thank you and you are welcome.
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