The phenomenon, called summer limb drop or sudden limb drop, often occurs on hot and windless summer days. Oak, Liquidambar, Pear and Ash trees are most prone to dropping large limbs on windless days. The immediate danger is to people, animals and cars parked under these trees.
Dr. Richard Harris, Professor of Landscape Horticulture at UC Davis, published an article on the topic in the Journal of Arboriculture in April 1983 describing this condition and its possible causes, and suggested steps that tree owners can take to reduce the hazard of summer branch drop.
Summer branch drop is not related to wind and often occurs in the afternoon on hot, calm days. Unlike most breaks due to wind, which occur where a branch attaches to the trunk, a break due to summer branch drop usually occurs 3 to 12 feet away from the trunk, along the length of the branch. The branches that break are usually long and horizontal, as opposed to upright, frequently extending to or beyond the average tree canopy. Once a tree has lost a limb due to summer branch drop, it is more likely to lose another.
Drought stress may somehow contribute to summer branch drop, but at this time, there is no generally accepted hypothesis that explains this occurrence. In California this type of limb failure occurs on both native and planted trees as well as in irrigated and un-irrigated landscapes. One possibility is that drought stress during a hot calm afternoon reduces the flow of water in the branch, causing the branch temperature and the concentration of ethylene to increase. Old wounds and decay hidden inside a limb (possibly resulting from improper pruning) occasionally contribute to branch drop, but this does not account for the majority of summer branch drop failures. Pruning that encourages uneven growth at the end of a limb can put tremendous stress on the limb due to the added weight of the new growth.
Although there is no guaranteed way to prevent summer branch drop, several things can be done to mitigate this hazard in oaks and other commonly affected tree species such as eucalyptus, elm, and ash. On mature trees, shorten and lighten long horizontal branches and open up the tree by thinning to healthy lateral branches to reduce branch weight. Inspect the tree for externally visible defects and prune out damaged or sickly low-vigor limbs that have decay or cavities. Although watering is required to keep most ornamental trees healthy in our hot summer climate, don’t forget that summer moisture can encourage oak root fungus and other oak pathogens that can kill oaks when the summer-watered area is within 10 feet of the trunk. Finally, do not park cars or place play structures, benches, or picnic tables beneath older, susceptible trees. Falling limbs can’t harm people (or property) if they aren’t under the tree.
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