Limb Failure Of Spontaneous Nature

Riparian trees notoriously shed limbs spontaneously.

Winter storms sometimes break limbs or topple trees. Such damage is no surprise during winter because that is when almost all windy weather happens here. Early storms during autumn might be more damaging because deciduous trees are less aerodynamic prior to defoliation. Nonetheless, falling trees and limb failure are typically associated with wind.

That is why spontaneous limb failure is such a surprise when it happens, typically during pleasantly mild or warm weather of spring or early summer. It is more likely without wind than with it. Humidity, although atypical here, is a contributing factor. Healthy trees within riparian situations or lawns are more susceptible than distressed trees in drier situations. 

Particular types of trees are more susceptible to spontaneous limb failure as well. Valley oak, coast live oak, sweetgum, carob, some pines and various eucalypti may shed limbs after an unusually rainy winter or an increase of irrigation. Riparian trees, such as willow, cottonwood, box elder and sycamore are notorious for shedding big limbs unexpectedly.

Spontaneous limb failure occurs if limbs are unable to support their own increasing foliar weight. Bloom can add significant weight too. Warmth promotes the vascular activity that increases foliar weight. Humidity and insufficient air circulation inhibit evapotranspiration (evaporation of foliar moisture), which typically compensates for increasing foliar weight.

As the terminology implies, spontaneous limb failure occurs suddenly, and often without warning. It is therefore potentially very dangerous. It is common among limbs that exhibit no prior structural deficiency. Even experienced and educated arborists who are familiar with vulnerable tree species can not identify and mitigate all potentially hazardous limbs. 

Arborists often suggest pruning to limit the weight of trees that are innately susceptible to spontaneous limb failure. However, limbs that are already sagging from their own weight are risky to engage. Most damage occurs in spring and early summer. Fruit trees that are too productive can succumb to the weight of fruit as it ripens through summer or autumn.


This is a bit more than just slightly inconvenient. The trail continues forward from here, with another trail up the stairs to the right.

It is that time of year. Warming weather accelerates vascular activity, which makes foliage heavier. If evapotranspiration is inhibited by humidity and a lack of wind, the foliage can get too heavy to be supported by the trees that produce it. All that increasing weight can bring down big limbs or entire trees at the most unexpected times. The process is spontaneous limb failure.

By ‘unexpected’, I mean that it happens when there is no wind. It is startling because broken limbs and fallen trees are typically associated with wind rather than a lack of it. Gentle wind actually accelerates evapotranspiration, which relives affected vegetation of some of its weight and susceptibility to spontaneous limb failure. Aridity helps too, by absorbing more moisture.

Of course, even a gentle breeze at the wrong time can have disastrous results for vegetation that is already about to succumb to spontaneous limb failure. I suspect that is what happened here, since the air was not completely still at the time. It was just a bit warmer than it had been, and slightly more humid than typical. It is too late and pointless to analyze the situation now.

This is the top of the stairs that are to the right in the picture above.

At about noon on Thursday, someone who works nearby alerted me to the sound a a big tree falling. I was in the same neighborhood, but was driving by with the radio on. The tree is precisely where I was told it would be. No one was nearby when it fell. Damage was originally minimal, with a portion of trail displaced by roots, and a rail on a bridge crushed by the trunk.

By ‘originally’, I mean that this was not the extent of the damage. After barricading the trails and road leading to the site, and leaving, we heard another loud crash from the same location as a bay tree that had been leaning against the already fallen fir tree collapsed in pieces on top of the whole mess. Fortunately, the damage to the bridge, although worse, is not too terribly bad.

There is more timber here than lumber. The short section of main trunk is severely fractures. The double trunks beyond are not as big as they look.

Spontaneous Limb Failure Is Real

90605thumbIt 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.