NEVADA CITY, Calif. August 26, 2019 – First, you hear a ripping, tearing sound. Then, a whoosh and the thud of a heavy object hitting the ground. A large, healthy-looking oak branch has suddenly dropped, most likely from a mature tree. 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. A secondary danger is limbs falling on power lines, causing outages – or worse – fires.
Scientists and arborists point to several possible causes for sudden limb drop. Drought stress can cause an increase in ethylene concentration, which combined with reduced evapotranspiration weakens a limb. Another possible cause are injuries to the tree, specifically at the fork of the branch, during winter. Fissures accumulate water, freezing water damages the fork and the limb is weakened. Another possible explanation is high moisture within the canopy, adding excess weight to the branches. Wetwood bacteria are also often cited as a possible cause.
Most limbs dropping are horizontal branches, sometimes extending beyond the tree’s canopy, often with an upsweep at the end. The downed limbs often show no obvious defects or rot on the inside.
Local arborist Zeno Acton is very familiar with the issue. He recommends regular maintenance especially for trees close to homes and, if needed, pruning by an ISA-certified arborist. An inspection by an arborist can diagnose possible weakness in a tree and help you maintain the health and safety of your landscape.
Sudden limb drop is a frequent cause of power outages in the Foothills during summer. Stay away from downed power lines. Treat all downed power lines as if they are energized and extremely dangerous. Keep yourself and others well away from them and immediately call 911, then notify PG&E’s 24-hour emergency and customer service line at 1-800-743-5002.