Summer Branch Drop

On the 12th of July, unexpectedly and without warning, a huge branch fell from one of our mature beech trees. It was on a very hot and calm afternoon. Less than an hour earlier two children had been playing on the swing underneath the trees. The branch that fell brought another with it. Altogether they must have weighed at least a ton. I'd never heard of this phenomenon until it happened here at Springhill.


We were having a barbecue and there were a good few people around. When we heard the noise, a loud sharp crack, followed by a creaking and a loud crash as the branches hit the ground, I thought at first that the tree house had come down. Almost everyone made a dash for the trees because no one was sure where Ben was. He had been playing up there for most of the day with another child. It was a big relief when we saw that he was fine. In fact, he had seen the branch drop and was the first on the scene. He whooped, "Bert is going to be so happy! Look at all that firewood!"


One of our guests had heard of this happening. He had worked in a government department that dealt with tree preservation orders. He told us that sudden branch falls in hot, still weather is something that can happen to old trees.


I checked it out later.



Ed Perry, Farm Advisor Stanislaus County
University of California, U.S. Department of Agriculture and Stanislaus County


Over the last couple of weeks a phenomenon known as “summer branch drop” has been apparent throughout the area. Also called “sudden limb failure,” the phenomenon occurs during periods of very hot weather when apparently sound, relatively large limbs break out of large, mature shade trees. Since most people think that branches only break during windy conditions, a large branch crashing to the ground on a hot and calm day causes some excitement.


Branches that fail due to summer branch drop are usually long and horizontal, rather than more upright. Oddly enough, many times the break occurs along the length of the branch somewhere, rather than at its point of attachment to the trunk. While some broken branches may have wounds or areas of wood decay, many that drop are free of any obvious defects and appear sound. Therefore, summer branch drop is very difficult to predict.


There is still no generally accepted explanation for the phenomenon. However, most tree experts believe lack of adequate soil moisture, or drought stress, is to blame. This is a bit difficult to explain, since branches actually become lighter during hot weather as they lose more water from leaves than they gain from the root system. Studies have shown that branches actually shrink and rise during summer afternoons. Another theory is that water stress causes the concentration of ethylene, a plant hormone, to increase. Ethylene is a gas produced by all plants that effects all stages of plant growth and development, including processes involved in cell aging. There is a possibility that increases in ethylene could dissolve the cementation of cell walls in the wood, causing the branch to break. Others suggest that internal cracks in large branches, caused by wounds or improper pruning, eventually spread outward, causing the branch to fracture.






The phenomenon is less common in the British Isles but it does happen. The Arboricultural Association said it was aware of a number of reports of branches dropping off trees, an event it says is associated with prolonged hot spells.


Paul Smith, a technical officer with the organisation, said: “Certainly there’s a clear relationship between the weather conditions and the frequency of summer branch drop.

The indications are that it’s to do with water stresses within very large, often overextended or elongated limbs.












That is how close the biggest branch was to the swing. Had anyone been using the swing when the branch came down, they would have been very fortunate to escape serious or mortal injury.







There was no obvious sign of rot or decay. The part of the branch where it broke from the trunk was very wet beneath the bark.












I have always thought that our mature beech trees were the very best thing about this property. I feel differently now. A little more anxious. Some experts say that when sudden branch drop occurs in a tree, there is an increased chance that it will happen again. The branch that collapsed was not the only large, extended branch on that tree. The swing is on another. With great trees come great responsibilities. Perhaps it is time to call in the experts.