Sudden Branch Drop Phenomena.

Branches fail because of defects and unusually heavy loads of foliage but there is another kind of failure known as "summer limb drop," "high temperature limb drop" and most recently as "sudden branch drop."

On a windless, hot summer evening, a big limb suddenly breaks with a loud "crack" and crashes to the ground. Examination of the failed limb reveals no obvious external defects but the inner wood is "brash," broken bluntly, with no sharp splinters.

One of the characteristics that make sudden branch drop so worrisome is that the failed limbs usually show no obvious visual signs of cracks or decay. Around the world, sudden branch drop has been reported in many countries and many tree species. Numerous arborists have looked closely to find the causes and visual clues associated with it, hoping that known warning signs will help us take action to prevent damage and injuries.

Theories about the cause include ideas about branch movement and shrinkage, internal cracks, internal moisture changes, ethylene gas released inside of branches and microscopic changes in cell wall structure.

There is now a set of visible clues to help spot, from the ground, trees and tree limbs that fit the profile for sudden branch drop. The tree is usually a mature, slow-growing specimen. The limbs are often very large and horizontal with an upsweep toward the end, and the limbs usually extend beyond the main canopy of the tree.

Trees reported to be susceptible to summer branch drop.

Quercus spp.
Populus spp.
Salix spp.
Ulmus procera
Castanea sativa
Fagus sylvatica
Fraxinus excelsior
Aesculus hippocastanum
Genera most often reported in California
Eucalyptus Quercus
Ulmus Pinus
Cedrus
Fraxinus
Platanus
Species also reported in California.
Ailanthus altissima
Erythrina caffra
Ficus microcarpa
Olea europaea
Grevillea robusta
Sequoiadendron giganteum
Sophora japonica

To reduce the risks associated with sudden branch drop where the warning signs have been spotted, arborists and tree owners should consider the following measures:

1. Prune at-risk limbs for crown thinning and reduction. Quite often, such limbs can be pruned judiciously, reducing weight without disfiguring the limb.

2. Prune the limb and then install a cable system to limit the motion and share the load with other limbs in the same tree or even an adjoining tree. Keep in mind that cable systems become an integral part of the tree and must be checked about every three years.

3. Prune to remove the at-risk limb if the first two options are not practical.

4. Restrict activities near the tree. If there is no target there is no hazard.

5. If options 1 through 4 are not practical, and the owner is not willing to accept some level of risk, remove the tree.

This brings us to a crucial point: accepting risk. To paraphrase an "Arborist's Disclosure Statement" tree owners must be aware that tree and limb failures are not always predictable. Arborists cannot always detect conditions that lead to structural failures, and no one can guarantee that a tree will not fail. Trees are living organisms and they may fail in ways we do not fully understand. Sudden limb drop is a perfect example.

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Summer Branch Drop is a common syndrome that happens when trees are struggling to balance the water coming in and leaving the plant through transpiration. The Sick Tree Treatment will help.

Here is a link to more information on this tree quirk.

http://joa.isa-arbor.com/request.asp?JournalID=1&ArticleID=1877&Type=2