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.
Matt Merrill, a certified arborist and owner of Yuba Forest Restoration, explains, “I have observed sudden limb drop in person several times, it is quite the phenomenon. The failures I have observed have been on large overextended oak limbs with a lot of weight concentrated at the tips. This could be due to poor structure or improper pruning practices (lions-tailing) in the past. Generally, an arborist would have identified these limbs as having a higher potential for failure had it been inspected prior. I think the fact that it happens in the middle of a hot and still day, as opposed to a storm event is what what makes it feel so ‘unpredictable’.”
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.
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. Hint: Look up before you park at the river or a lake.
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.
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.